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The Project Gutenberg eBook of Research methods in ecology
This eBook is for the use of anyone anywhere in the United States and
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Title: Research methods in ecology
Author: Frederic E. Clements
Release date: April 18, 2024 [eBook #73420]
Language: English
Original publication: Lincoln: University Publishing Co, 1905
Other information and formats: www.gutenberg.org/ebooks/73420
Credits: Richard Tonsing and The Online Distributed Proofreading
Team at https://www.pgdp.net (This file was produced from
images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK RESEARCH
METHODS IN ECOLOGY ***
Transcriber’s Note:
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online at
www.gutenberg.org. If you are not located in the United States, you
will have to check the laws of the country where you are located
before using this eBook.
Title: Research methods in ecology
Author: Frederic E. Clements
Release date: April 18, 2024 [eBook #73420]
Language: English
Original publication: Lincoln: University Publishing Co, 1905
Other information and formats: www.gutenberg.org/ebooks/73420
Credits: Richard Tonsing and The Online Distributed Proofreading
Team at https://www.pgdp.net (This file was produced from
images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK RESEARCH
METHODS IN ECOLOGY ***
Transcriber’s Note:
Page 4
New original cover art included with this eBook is
granted to the public domain.
granted to the public domain.
Page 5
RESEARCH METHODS
IN
ECOLOGY
BY
FREDERIC EDWARD CLEMENTS, Ph.D.
ASSOCIATE PROFESSOR OF PLANT PHYSIOLOGY IN THE UNIVERSITY OF NEBRASKA
ILLUSTRATED
LINCOLN, NEBRASKA
The University Publishing Company
1905
IN
ECOLOGY
BY
FREDERIC EDWARD CLEMENTS, Ph.D.
ASSOCIATE PROFESSOR OF PLANT PHYSIOLOGY IN THE UNIVERSITY OF NEBRASKA
ILLUSTRATED
LINCOLN, NEBRASKA
The University Publishing Company
1905
Page 6
Copyright, 1905
By FREDERIC E. CLEMENTS AND IRVING S. CUTTER
All rights reserved
Press of Jacob North & Company
Lincoln, Nebraska
By FREDERIC E. CLEMENTS AND IRVING S. CUTTER
All rights reserved
Press of Jacob North & Company
Lincoln, Nebraska
Page 7
PREFACE
The present volume is intended as a handbook for investigators and for
advanced students of ecology, and not as a text-book of the subject. An
elementary text-book covering the same field, but adapted to the needs of
undergraduate students, is in preparation. The handbook is essentially an
account of the methods used by the author in his studies of the last eight
years, during which a serious attempt has been made to discover and to
correlate the fundamental points of view in the vast field of vegetation. No
endeavor is made to treat any portion of the subject exhaustively, since a
discussion of general methods and general principles is of much greater
value in the present condition of ecology. The somewhat unequal treatment
given the different subjects is due to the fact that it has been found possible
to develop some of these more rapidly than others. Finally, it must be
constantly kept in mind that ecology is still in a very plastic condition, and
in consequence, methods, fundamental principles, and matters of
nomenclature and terminology must be approached without prejudice in
order that the best possible development of this field may be attained.
Grateful acknowledgment for criticisms and suggestions is made to
Professor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound,
who have read the text. The author is under especial obligations to Doctor
Edith S. Clements for the drawings of leaf types, as well as for reading and
criticising the manuscript. Professor Goodwin D. Swezey, Professor of
Astronomy in the University of Nebraska, has kindly furnished much
material for the determination of the sun’s altitude, and consequent light
intensities, and has read the section devoted to light. Mr. George A.
Loveland, Director of the Nebraska Section of the U. S. Weather Bureau,
has contributed many helpful suggestions to the discussion of
meteorological instruments. To Nella Schlesinger, Alice Venters, and
The present volume is intended as a handbook for investigators and for
advanced students of ecology, and not as a text-book of the subject. An
elementary text-book covering the same field, but adapted to the needs of
undergraduate students, is in preparation. The handbook is essentially an
account of the methods used by the author in his studies of the last eight
years, during which a serious attempt has been made to discover and to
correlate the fundamental points of view in the vast field of vegetation. No
endeavor is made to treat any portion of the subject exhaustively, since a
discussion of general methods and general principles is of much greater
value in the present condition of ecology. The somewhat unequal treatment
given the different subjects is due to the fact that it has been found possible
to develop some of these more rapidly than others. Finally, it must be
constantly kept in mind that ecology is still in a very plastic condition, and
in consequence, methods, fundamental principles, and matters of
nomenclature and terminology must be approached without prejudice in
order that the best possible development of this field may be attained.
Grateful acknowledgment for criticisms and suggestions is made to
Professor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound,
who have read the text. The author is under especial obligations to Doctor
Edith S. Clements for the drawings of leaf types, as well as for reading and
criticising the manuscript. Professor Goodwin D. Swezey, Professor of
Astronomy in the University of Nebraska, has kindly furnished much
material for the determination of the sun’s altitude, and consequent light
intensities, and has read the section devoted to light. Mr. George A.
Loveland, Director of the Nebraska Section of the U. S. Weather Bureau,
has contributed many helpful suggestions to the discussion of
meteorological instruments. To Nella Schlesinger, Alice Venters, and
Page 8
George L. Fawcett, advanced students in experimental ecology, the author
is indebted for many experiments which have been used in the discussion of
adjustment and adaptation.
Acknowledgment is also made to the following for various cuts: Henry J.
Green, Brooklyn, New York; Julien P. Friez, Baltimore, Maryland; C. H.
Stoelting Co., Chicago, Illinois; Draper Manufacturing Co., New York city;
Gundlach-Manhattan Optical Co., Rochester, New York; Rochester Optical
Co., Rochester, New York; Bausch and Lomb Optical Co., Rochester, New
York.
FREDERIC EDWARD CLEMENTS.
The University of Nebraska,
May, 1905.
is indebted for many experiments which have been used in the discussion of
adjustment and adaptation.
Acknowledgment is also made to the following for various cuts: Henry J.
Green, Brooklyn, New York; Julien P. Friez, Baltimore, Maryland; C. H.
Stoelting Co., Chicago, Illinois; Draper Manufacturing Co., New York city;
Gundlach-Manhattan Optical Co., Rochester, New York; Rochester Optical
Co., Rochester, New York; Bausch and Lomb Optical Co., Rochester, New
York.
FREDERIC EDWARD CLEMENTS.
The University of Nebraska,
May, 1905.
Page 9
CONTENTS
Page 10
Chapter I. The Foundation of Ecology
THE NEED OF A SYSTEM
PAGE
1. The scope of ecology 1
2. Ecology and physiology 1
Historical Development
3. Geographical distribution 2
4. The plant formation 2
5. Plant succession 3
6. Ecological phytogeography 4
7. Experimental ecology 4
8. Ecology of the habitat 5
9. The evidence from historical development 6
Present Status of Ecology
10. The lack of special training 6
11. Descriptive ecology 7
12. The value of floristic 8
13. Reconnaissance and investigation 8
14. Resident investigation 9
15. The dangers of a restricted field 9
Applications of Ecology
16. The subjects touched by ecology 10
17. Physiology and pathology 11
18. Experimental evolution 11
19. Taxonomy 12
20. Forestry 14
21. Physiography 15
22. Soil physics 15
23. Zoogeography 15
24. Sociology 16
THE ESSENTIALS OF A SYSTEM
25. Cause and effect: habitat and plant 16
26. The place of function 17
Chapter II. The Habitat
THE NEED OF A SYSTEM
PAGE
1. The scope of ecology 1
2. Ecology and physiology 1
Historical Development
3. Geographical distribution 2
4. The plant formation 2
5. Plant succession 3
6. Ecological phytogeography 4
7. Experimental ecology 4
8. Ecology of the habitat 5
9. The evidence from historical development 6
Present Status of Ecology
10. The lack of special training 6
11. Descriptive ecology 7
12. The value of floristic 8
13. Reconnaissance and investigation 8
14. Resident investigation 9
15. The dangers of a restricted field 9
Applications of Ecology
16. The subjects touched by ecology 10
17. Physiology and pathology 11
18. Experimental evolution 11
19. Taxonomy 12
20. Forestry 14
21. Physiography 15
22. Soil physics 15
23. Zoogeography 15
24. Sociology 16
THE ESSENTIALS OF A SYSTEM
25. Cause and effect: habitat and plant 16
26. The place of function 17
Chapter II. The Habitat
Page 11
CONCEPT AND ANALYSIS
27. Definition of the habitat 18
28. Factors 18
Classification of Factors
29. The nature of factors 19
30. The influence of factors 19
Determination of Factors
31. The need of exact measurement 20
32. The value of meteorological methods 20
33. Habitat determination 21
34. Determinable and efficient differences 21
Instrumentation
35. Methods 22
36. Method of simple instruments 22
37. Method of automatic instruments 23
38. Combined methods 23
CONSTRUCTION AND USE OF INSTRUMENTS
39. The selection of instruments 24
Water-content
40. Value of different instruments 25
Geotome methods
41. The geotome 25
42. Soil borers 26
43. Taking samples of soil 26
44. Weighing 27
45. Computation 28
46. Time and location of readings 28
47. Location of readings 29
48. Depth of samples 30
49. Check and control instruments 30
Physical and Physiological Water
50. The availability of soil water 30
51. Terms 31
52. Chresard determination under control 32
53. Chresard readings in the field 33
54. Chresard values of different soils 34
Records and Results
55. The field record 35
27. Definition of the habitat 18
28. Factors 18
Classification of Factors
29. The nature of factors 19
30. The influence of factors 19
Determination of Factors
31. The need of exact measurement 20
32. The value of meteorological methods 20
33. Habitat determination 21
34. Determinable and efficient differences 21
Instrumentation
35. Methods 22
36. Method of simple instruments 22
37. Method of automatic instruments 23
38. Combined methods 23
CONSTRUCTION AND USE OF INSTRUMENTS
39. The selection of instruments 24
Water-content
40. Value of different instruments 25
Geotome methods
41. The geotome 25
42. Soil borers 26
43. Taking samples of soil 26
44. Weighing 27
45. Computation 28
46. Time and location of readings 28
47. Location of readings 29
48. Depth of samples 30
49. Check and control instruments 30
Physical and Physiological Water
50. The availability of soil water 30
51. Terms 31
52. Chresard determination under control 32
53. Chresard readings in the field 33
54. Chresard values of different soils 34
Records and Results
55. The field record 35
Page 12
56. The permanent record 36
57. Sums and means 36
58. Curves 37
Humidity
59. Instruments 37
Psychrometers
60. Kinds 37
61. The sling psychrometer 38
62. Readings 39
63. Cog psychrometer 39
64. Construction and use 40
65. Hygrometers 40
Psychrographs
66. The Draper psychrograph 41
67. Placing the instrument 42
68. Regulating and operating the instrument 43
69. The weekly visit 44
Humidity Readings and Records
70. The time of readings 44
71. Place and height 45
72. Check instruments 45
73. Humidity tables 46
74. Sums, means, and curves 47
Conversion scale for temperatures
75. Records 48
Light
76. Methods 48
The Photometer
77. Construction 49
78. Filling the photometer 50
79. Making readings 50
80. The Dawson-Lander sun recorder 51
81. The selagraph 52
Standards
82. Use 53
83. Making a standard 53
84. Kinds of standards 54
Readings
85. Time 55
Chart for determining sun’s altitude 57
86. Table for determining apparent noon 58
57. Sums and means 36
58. Curves 37
Humidity
59. Instruments 37
Psychrometers
60. Kinds 37
61. The sling psychrometer 38
62. Readings 39
63. Cog psychrometer 39
64. Construction and use 40
65. Hygrometers 40
Psychrographs
66. The Draper psychrograph 41
67. Placing the instrument 42
68. Regulating and operating the instrument 43
69. The weekly visit 44
Humidity Readings and Records
70. The time of readings 44
71. Place and height 45
72. Check instruments 45
73. Humidity tables 46
74. Sums, means, and curves 47
Conversion scale for temperatures
75. Records 48
Light
76. Methods 48
The Photometer
77. Construction 49
78. Filling the photometer 50
79. Making readings 50
80. The Dawson-Lander sun recorder 51
81. The selagraph 52
Standards
82. Use 53
83. Making a standard 53
84. Kinds of standards 54
Readings
85. Time 55
Chart for determining sun’s altitude 57
86. Table for determining apparent noon 58
Page 13
87. Place 59
Table of intensity at various angles 60
Reflected and Absorbed Light
88. The fate of incident light 60
89. Methods of determination 61
90. Leaf and epidermis prints 62
Expression of Results
91. Light records 63
92. Light sums, means and curves 63
Temperature
93. 64
Thermometers
94. Air thermometers 64
95. Soil thermometers 64
96. Maximum-minimum thermometers 65
97. Radiation thermometers 67
98. Thermographs 67
Readings
99. Time 69
100. Place and height 70
Expression of Results
101. Temperature records 70
102. Temperature sums and means 70
103. Temperature curves 71
104. Plant temperatures 71
Precipitation
105. General relations 72
106. The rain gauge 73
107. Precipitation records 74
Wind
108. Value of readings 74
109. The anemometer 75
110. Records 76
Soil
111. Soil as a factor 76
112. The value of soil surveys 77
113. The origin of soils 77
114. The structure of soils 78
115. Mechanical analysis 79
116. Kinds of soils 79
117. The chemical nature of soils 80
Table of intensity at various angles 60
Reflected and Absorbed Light
88. The fate of incident light 60
89. Methods of determination 61
90. Leaf and epidermis prints 62
Expression of Results
91. Light records 63
92. Light sums, means and curves 63
Temperature
93. 64
Thermometers
94. Air thermometers 64
95. Soil thermometers 64
96. Maximum-minimum thermometers 65
97. Radiation thermometers 67
98. Thermographs 67
Readings
99. Time 69
100. Place and height 70
Expression of Results
101. Temperature records 70
102. Temperature sums and means 70
103. Temperature curves 71
104. Plant temperatures 71
Precipitation
105. General relations 72
106. The rain gauge 73
107. Precipitation records 74
Wind
108. Value of readings 74
109. The anemometer 75
110. Records 76
Soil
111. Soil as a factor 76
112. The value of soil surveys 77
113. The origin of soils 77
114. The structure of soils 78
115. Mechanical analysis 79
116. Kinds of soils 79
117. The chemical nature of soils 80
Page 14
Physiography
118. Factors 80
Altitude
119. Analysis into factors 81
120. The barometer 82
Slope
121. Concept 83
122. The clinometer 83
123. The trechometer 84
Exposure
124. Exposure 85
125. Surface 85
126. Record of physiographic factors 86
127. Topography 86
Biotic Factors
128. Influence and importance 86
129. Animals 87
130. Plants 87
METHODS OF HABITAT INVESTIGATION
131. 88
Method of Simple Instruments
132. Choice of stations 88
133. Time of readings 89
134. Details of the method 89
135. Records 91
Method of Ecograph Batteries
136. 92
Expression of Physical Factor Results
137. The form of results 94
Factor Records
138. 94
Factor Curves
139. Plotting 95
140. Kinds of curves 96
141. Combinations of curves 96
142. The amplitude of curves 98
Factor Means and Sums
143. 98
118. Factors 80
Altitude
119. Analysis into factors 81
120. The barometer 82
Slope
121. Concept 83
122. The clinometer 83
123. The trechometer 84
Exposure
124. Exposure 85
125. Surface 85
126. Record of physiographic factors 86
127. Topography 86
Biotic Factors
128. Influence and importance 86
129. Animals 87
130. Plants 87
METHODS OF HABITAT INVESTIGATION
131. 88
Method of Simple Instruments
132. Choice of stations 88
133. Time of readings 89
134. Details of the method 89
135. Records 91
Method of Ecograph Batteries
136. 92
Expression of Physical Factor Results
137. The form of results 94
Factor Records
138. 94
Factor Curves
139. Plotting 95
140. Kinds of curves 96
141. Combinations of curves 96
142. The amplitude of curves 98
Factor Means and Sums
143. 98
Page 15
Chapter III. The Plant
STIMULUS AND RESPONSE
General Relations
144. The nature of stimuli 100
145. The kinds of stimuli 100
146. The nature of response 101
147. Adjustment and adaptation 102
148. The measurement of response 103
149. Plasticity and fixity 104
150. The law of extremes 105
151. The method of working hypotheses 106
Hydroharmose
Adjustment
152. Water as a stimulus 107
153. The influence of other factors upon water 107
154. Response 108
155. The measurement of absorption 109
156. The quantitative relation of absorption and transpiration 111
157. Measurement of transpiration 113
158. Field methods 114
159. Expression of results 116
160. Coefficient of transpiration 117
Adaptation
161. Modifications due to water stimuli 118
162. Modifications due to a small water supply 118
163. The decrease of water loss 118
164. The increase of water supply 121
165. Modifications due to an excessive water supply 121
166. Plant types 122
167. Xerophytic types 122
168. Types of leaf xerophytes 123
169. Types of stem xerophytes 125
170. Bog plants 126
171. Hydrophytic types 127
Photoharmose
Adjustment
172. Light as a stimulus 129
173. The reception of light stimuli 131
STIMULUS AND RESPONSE
General Relations
144. The nature of stimuli 100
145. The kinds of stimuli 100
146. The nature of response 101
147. Adjustment and adaptation 102
148. The measurement of response 103
149. Plasticity and fixity 104
150. The law of extremes 105
151. The method of working hypotheses 106
Hydroharmose
Adjustment
152. Water as a stimulus 107
153. The influence of other factors upon water 107
154. Response 108
155. The measurement of absorption 109
156. The quantitative relation of absorption and transpiration 111
157. Measurement of transpiration 113
158. Field methods 114
159. Expression of results 116
160. Coefficient of transpiration 117
Adaptation
161. Modifications due to water stimuli 118
162. Modifications due to a small water supply 118
163. The decrease of water loss 118
164. The increase of water supply 121
165. Modifications due to an excessive water supply 121
166. Plant types 122
167. Xerophytic types 122
168. Types of leaf xerophytes 123
169. Types of stem xerophytes 125
170. Bog plants 126
171. Hydrophytic types 127
Photoharmose
Adjustment
172. Light as a stimulus 129
173. The reception of light stimuli 131
Page 16
174. The response of the chloroplast 132
175. Aeration and translocation 134
176. The measurement of responses to light 135
Adaptation
177. Influence of chloroplasts upon form and structure 138
178. Form of leaves and stems 139
179. Modification of the epidermis 140
180. The differentiation of the chlorenchym 142
181. Types of leaves 144
182. Heliophytes and sciophytes 144
EXPERIMENTAL EVOLUTION
183. Scope 145
184. Fundamental lines of inquiry 146
185. Ancestral form and structure 146
186. Variation and mutation 147
187. Methods 149
Method of Natural Experiment
188. Selection of species 149
189. Determination of factors 151
190. Method of record 152
Method of Habitat Cultures
191. Scope and advantages 153
192. Methods 153
193. Transfer 154
194. Modification of the habitat 156
Method of Control Cultures
195. Scope and procedure 157
196. Water-content series 158
197. Light series 160
Chapter IV. The Formation
METHODS OF INVESTIGATION AND RECORD
198. The need of exact methods 161
Quadrats
199. Uses 161
200. Possible objections 163
175. Aeration and translocation 134
176. The measurement of responses to light 135
Adaptation
177. Influence of chloroplasts upon form and structure 138
178. Form of leaves and stems 139
179. Modification of the epidermis 140
180. The differentiation of the chlorenchym 142
181. Types of leaves 144
182. Heliophytes and sciophytes 144
EXPERIMENTAL EVOLUTION
183. Scope 145
184. Fundamental lines of inquiry 146
185. Ancestral form and structure 146
186. Variation and mutation 147
187. Methods 149
Method of Natural Experiment
188. Selection of species 149
189. Determination of factors 151
190. Method of record 152
Method of Habitat Cultures
191. Scope and advantages 153
192. Methods 153
193. Transfer 154
194. Modification of the habitat 156
Method of Control Cultures
195. Scope and procedure 157
196. Water-content series 158
197. Light series 160
Chapter IV. The Formation
METHODS OF INVESTIGATION AND RECORD
198. The need of exact methods 161
Quadrats
199. Uses 161
200. Possible objections 163
Page 17
Kinds of Quadrats and Their Use
201. Size and kinds 164
202. Tapes and stakes 164
203. Locating quadrats 165
The List Quadrat
204. Description 165
205. Manner of use 166
206. Table of abundance 166
The Chart Quadrat
207. Description and use 167
208. The chart 168
209. Mapping 168
210. Factors and photographs 170
The Permanent Quadrat
211. Description and uses 170
212. Manner of use 172
The Denuded Quadrat
213. Description 173
214. Methods of denuding and recording 174
215. Physical factors 175
Aquatic Quadrats
216. Scope 175
Transects
217. The transect 176
The Line Transect
218. Description and method 176
219. The location and size 177
The Belt Transect
220. Details 178
The Permanent Transect
221. Advantages 179
222. Details 179
The Denuded Transect
223. 180
The Layer Transect
224. 180
Ecotone Charts
225. 181
The Migration Circle
226. Purpose 182
227. Location and method 182
201. Size and kinds 164
202. Tapes and stakes 164
203. Locating quadrats 165
The List Quadrat
204. Description 165
205. Manner of use 166
206. Table of abundance 166
The Chart Quadrat
207. Description and use 167
208. The chart 168
209. Mapping 168
210. Factors and photographs 170
The Permanent Quadrat
211. Description and uses 170
212. Manner of use 172
The Denuded Quadrat
213. Description 173
214. Methods of denuding and recording 174
215. Physical factors 175
Aquatic Quadrats
216. Scope 175
Transects
217. The transect 176
The Line Transect
218. Description and method 176
219. The location and size 177
The Belt Transect
220. Details 178
The Permanent Transect
221. Advantages 179
222. Details 179
The Denuded Transect
223. 180
The Layer Transect
224. 180
Ecotone Charts
225. 181
The Migration Circle
226. Purpose 182
227. Location and method 182
Page 18
228. The denuded circle 183
229. Photographs 183
Cartography
230. Value of cartographic methods 183
231. Standard scale 184
232. Color scheme 184
233. Formation and vegetation maps 185
234. Continental maps 187
Photography
235. 188
236. The camera and its accessories 188
237. The choice of a camera 190
238. The use of the camera 191
239. The sequence of details 192
240. The time of exposure 193
241. Developing 194
242. Finishing 195
Formation and Succession Herbaria
243. Concept and purpose 196
244. Details of collecting 197
245. Arrangement 197
246. Succession herbaria 198
DEVELOPMENT AND STRUCTURE
247. Vegetation an organism 199
248. Vegetation essentially dynamic 199
249. Functions and structures 199
Association
250. Concept 200
251. Causes 201
252. Aggregation 203
Kinds of Association
253. Categories 204
254. Stratum association 204
255. Ground association 205
256. Species guild association 206
257. Light association 206
258. Water-content association 208
THE DEVELOPMENT OF THE FORMATION
229. Photographs 183
Cartography
230. Value of cartographic methods 183
231. Standard scale 184
232. Color scheme 184
233. Formation and vegetation maps 185
234. Continental maps 187
Photography
235. 188
236. The camera and its accessories 188
237. The choice of a camera 190
238. The use of the camera 191
239. The sequence of details 192
240. The time of exposure 193
241. Developing 194
242. Finishing 195
Formation and Succession Herbaria
243. Concept and purpose 196
244. Details of collecting 197
245. Arrangement 197
246. Succession herbaria 198
DEVELOPMENT AND STRUCTURE
247. Vegetation an organism 199
248. Vegetation essentially dynamic 199
249. Functions and structures 199
Association
250. Concept 200
251. Causes 201
252. Aggregation 203
Kinds of Association
253. Categories 204
254. Stratum association 204
255. Ground association 205
256. Species guild association 206
257. Light association 206
258. Water-content association 208
THE DEVELOPMENT OF THE FORMATION
Page 19
259. 210
Invasion
260. 210
Migration
261. 210
262. Mobility 211
263. Organs for dissemination 211
264. Contrivances for dissemination 212
265. Position of disseminules 214
266. Seed production 215
267. Agents of migration 216
268. The direction of migration 219
Ecesis
269. Concept 220
270. Germination of the seed 221
271. Adjustment to the habitat 223
Barriers
272. Concept 224
273. Physical barriers 225
274. Biological barriers 225
275. Influence of barriers 226
Endemism
276. Concept 227
277. Causes 228
278. Significance 228
Polyphylesis and Polygenesis
279. Concept 230
280. Proofs of polygenesis 231
281. Origin by polyphylesis 232
Kinds of Invasion
282. Continuous and intermittent invasion 234
283. Complete and partial invasion 235
284. Permanent and temporary invasion 235
Manner of Invasion
285. Entrance into the habitat 236
286. Influence of levels 238
Investigation of Invasion
287. 239
Succession
288. Concept 239
Invasion
260. 210
Migration
261. 210
262. Mobility 211
263. Organs for dissemination 211
264. Contrivances for dissemination 212
265. Position of disseminules 214
266. Seed production 215
267. Agents of migration 216
268. The direction of migration 219
Ecesis
269. Concept 220
270. Germination of the seed 221
271. Adjustment to the habitat 223
Barriers
272. Concept 224
273. Physical barriers 225
274. Biological barriers 225
275. Influence of barriers 226
Endemism
276. Concept 227
277. Causes 228
278. Significance 228
Polyphylesis and Polygenesis
279. Concept 230
280. Proofs of polygenesis 231
281. Origin by polyphylesis 232
Kinds of Invasion
282. Continuous and intermittent invasion 234
283. Complete and partial invasion 235
284. Permanent and temporary invasion 235
Manner of Invasion
285. Entrance into the habitat 236
286. Influence of levels 238
Investigation of Invasion
287. 239
Succession
288. Concept 239
Page 20
289. Kinds of succession 240
Primary Successions
290. 241
291. Succession through elevation 241
292. Succession through volcanic action 242
293. Weathering 243
294. Succession in residuary soils 243
295. Succession in colluvial soils 244
296. Succession in alluvial soils 245
297. Succession in aeolian soils 246
298. Succession in glacial soils 247
Secondary Successions
299. 247
300. Succession in eroded soils 247
301. Succession in flooded soils 248
302. Succession by subsidence 249
303. Succession in land slips 249
304. Succession in drained or dried soils 249
305. Succession by animal agency 250
306. Succession by human agency 250
307. Succession in burned areas 251
308. Succession in lumbered areas 252
309. Succession by cultivation 253
310. Succession by drainage 253
311. Succession by irrigation 253
312. Anomalous successions 254
313. Perfect and imperfect successions 254
314. Stabilization 255
Causes and Reactions
315. 256
316. Succession by preventing weathering 257
317. Succession by binding aeolian soils 258
318. Succession by reducing run-off and erosion 259
319. Succession by filling with silt and plant remains 260
320. Succession by enriching the soil 261
321. Succession by exhausting the soil 262
322. Succession by the accumulation of humus 263
323. Succession by modifying atmospheric factors 264
Laws of Succession
324. 264
Classification and Nomenclature
Primary Successions
290. 241
291. Succession through elevation 241
292. Succession through volcanic action 242
293. Weathering 243
294. Succession in residuary soils 243
295. Succession in colluvial soils 244
296. Succession in alluvial soils 245
297. Succession in aeolian soils 246
298. Succession in glacial soils 247
Secondary Successions
299. 247
300. Succession in eroded soils 247
301. Succession in flooded soils 248
302. Succession by subsidence 249
303. Succession in land slips 249
304. Succession in drained or dried soils 249
305. Succession by animal agency 250
306. Succession by human agency 250
307. Succession in burned areas 251
308. Succession in lumbered areas 252
309. Succession by cultivation 253
310. Succession by drainage 253
311. Succession by irrigation 253
312. Anomalous successions 254
313. Perfect and imperfect successions 254
314. Stabilization 255
Causes and Reactions
315. 256
316. Succession by preventing weathering 257
317. Succession by binding aeolian soils 258
318. Succession by reducing run-off and erosion 259
319. Succession by filling with silt and plant remains 260
320. Succession by enriching the soil 261
321. Succession by exhausting the soil 262
322. Succession by the accumulation of humus 263
323. Succession by modifying atmospheric factors 264
Laws of Succession
324. 264
Classification and Nomenclature
Page 21
325. Basis 267
326. Nomenclature 267
327. Illustrations 270
Investigation of Succession
328. General rules 270
329. Method of alternating stages 271
330. The relict method 272
THE STRUCTURE OF THE FORMATION
331. 274
Zonation
332. Concept 274
Causes of Zonation
333. Growth 275
334. Reactions 276
335. Physical factors 276
336. Physiographic symmetry 278
Kinds of Zonation
337. 279
338. Radial zonation 280
339. Bilateral zonation 280
340. Vertical zonation 280
341. Vegetation zones 281
Alternation
342. Concept 283
343. Causes 284
344. Competition 285
345. Kinds of alternation 289
The Formation in Detail
346. The rank of the formation 292
347. The parts of a formation 295
348. Nomenclature of the divisions 299
349. The investigation of a particular formation 299
Classification and Relationship
350. Basis 300
351. Habitat classification 301
352. Nomenclature 302
353. Developmental classification 304
354. Regional classification 304
355. Mixed formations 304
326. Nomenclature 267
327. Illustrations 270
Investigation of Succession
328. General rules 270
329. Method of alternating stages 271
330. The relict method 272
THE STRUCTURE OF THE FORMATION
331. 274
Zonation
332. Concept 274
Causes of Zonation
333. Growth 275
334. Reactions 276
335. Physical factors 276
336. Physiographic symmetry 278
Kinds of Zonation
337. 279
338. Radial zonation 280
339. Bilateral zonation 280
340. Vertical zonation 280
341. Vegetation zones 281
Alternation
342. Concept 283
343. Causes 284
344. Competition 285
345. Kinds of alternation 289
The Formation in Detail
346. The rank of the formation 292
347. The parts of a formation 295
348. Nomenclature of the divisions 299
349. The investigation of a particular formation 299
Classification and Relationship
350. Basis 300
351. Habitat classification 301
352. Nomenclature 302
353. Developmental classification 304
354. Regional classification 304
355. Mixed formations 304
Page 22
EXPERIMENTAL VEGETATION
356. Scope and methods 306
Method of Natural Habitats
357. Natural experiments 307
Method of Artificial Habitats
358. Modification of habitat 307
359. Denuding 308
360. Modification of the formation by transfer 309
Method of Control Habitats
361. Competition cultures 310
362. Details of culture methods 311
Glossary 314
Bibliography 324
356. Scope and methods 306
Method of Natural Habitats
357. Natural experiments 307
Method of Artificial Habitats
358. Modification of habitat 307
359. Denuding 308
360. Modification of the formation by transfer 309
Method of Control Habitats
361. Competition cultures 310
362. Details of culture methods 311
Glossary 314
Bibliography 324
Page 23
RESEARCH METHODS IN ECOLOGY
Page 24
CHAPTER I. THE FOUNDATION OF
ECOLOGY
The Need of a System
1. The scope of ecology. The clue to the field of ecology is found in the
Greek word, οἲκος, home. The point of view in the following treatise is
constantly that which is inherent in the term itself. Ecology is therefore
considered the dominant theme in the study of plants, indeed, as the central
and vital part of botany. This statement may at first appear startling, if not
unfounded, but mature reflection will show that all the questions of
botanical science lead sooner or later to the two ultimate facts: plant and
habitat. The essential truth of this has been much obscured by detached
methods of study in physiology, morphology, and histology, which are too
often treated as independent fields. These have suffered incomplete and
unsymmetric development in consequence of extreme specialistic
tendencies. Analytic methods have dominated research to the exclusion of
synthetic ones, which, in a greatly diversified field, must be final, if
botanical knowledge is something to be systematized and not merely
catalogued. Physiology in particular has suffered at the hands of detached
specialists. Originally conceived as an inquiry into the origin and nature of
plants, it has been developed strictly as a study of plant activities. It all but
ignores the physical factors that control function, and the organs and tissues
that reflect it.
2. Ecology and physiology. There can be little question in regard to the
essential identity of physiology and ecology. This is evident when it is
clearly seen that the present difference between the two fields is superficial.
Ecology has been largely the descriptive study of vegetation; physiology
has concerned itself with function; but, when carefully analyzed, both are
seen to rest upon the same foundation. In each, the development is
ECOLOGY
The Need of a System
1. The scope of ecology. The clue to the field of ecology is found in the
Greek word, οἲκος, home. The point of view in the following treatise is
constantly that which is inherent in the term itself. Ecology is therefore
considered the dominant theme in the study of plants, indeed, as the central
and vital part of botany. This statement may at first appear startling, if not
unfounded, but mature reflection will show that all the questions of
botanical science lead sooner or later to the two ultimate facts: plant and
habitat. The essential truth of this has been much obscured by detached
methods of study in physiology, morphology, and histology, which are too
often treated as independent fields. These have suffered incomplete and
unsymmetric development in consequence of extreme specialistic
tendencies. Analytic methods have dominated research to the exclusion of
synthetic ones, which, in a greatly diversified field, must be final, if
botanical knowledge is something to be systematized and not merely
catalogued. Physiology in particular has suffered at the hands of detached
specialists. Originally conceived as an inquiry into the origin and nature of
plants, it has been developed strictly as a study of plant activities. It all but
ignores the physical factors that control function, and the organs and tissues
that reflect it.
2. Ecology and physiology. There can be little question in regard to the
essential identity of physiology and ecology. This is evident when it is
clearly seen that the present difference between the two fields is superficial.
Ecology has been largely the descriptive study of vegetation; physiology
has concerned itself with function; but, when carefully analyzed, both are
seen to rest upon the same foundation. In each, the development is
Page 25
incomplete: ecology has so far been merely superficial, physiology too
highly specialized. The one is chaotic and unsystematized, the other too
often a minute study of function under abnormal circumstances. The
greatest need of the former is the introduction of method and system, of the
latter, a broadening of scope and new objectives. The growing recognition
of the identity of the two makes it desirable to anticipate their final merging,
and to formulate a system that will combine the good in each, and at the
same time eliminate superficial and extreme tendencies. In this connection,
it becomes necessary to point out to ecologist and physiologist alike that,
while they have been working on the confines of the same great field, each
must familiarize himself with the work and methods of the other, before his
preparation is complete. Both must broaden their horizons, and rearrange
their views. The ecologist is sadly in need of the more intimate and exact
methods of the physiologist: the latter must take his experiments into the
field, and must recognize more fully that function is but the middleman
between habitat and plant. It seems probable that the final name for the
whole field will be physiology, although the term ecology has distinct
advantages of brevity and of meaning. In this event, however, it should be
clearly recognized that, although the name remains the same, the field has
become greatly broadened by new viewpoints and new methods.
highly specialized. The one is chaotic and unsystematized, the other too
often a minute study of function under abnormal circumstances. The
greatest need of the former is the introduction of method and system, of the
latter, a broadening of scope and new objectives. The growing recognition
of the identity of the two makes it desirable to anticipate their final merging,
and to formulate a system that will combine the good in each, and at the
same time eliminate superficial and extreme tendencies. In this connection,
it becomes necessary to point out to ecologist and physiologist alike that,
while they have been working on the confines of the same great field, each
must familiarize himself with the work and methods of the other, before his
preparation is complete. Both must broaden their horizons, and rearrange
their views. The ecologist is sadly in need of the more intimate and exact
methods of the physiologist: the latter must take his experiments into the
field, and must recognize more fully that function is but the middleman
between habitat and plant. It seems probable that the final name for the
whole field will be physiology, although the term ecology has distinct
advantages of brevity and of meaning. In this event, however, it should be
clearly recognized that, although the name remains the same, the field has
become greatly broadened by new viewpoints and new methods.
Page 26
HISTORICAL DEVELOPMENT
3. Geographical distribution. The systematic analysis of the great field
of ecology is essential to its proper development in the future. A glance at
its history shows that, while a number of essential points of attack have
been discovered, only one or two of these have been organized, and that
there is still an almost entire lack of correlation and coordination between
these. The earliest and simplest development of the subject was concerned
with the distribution of plants. This was at first merely an offshoot of
taxonomy, and, in spite of the work of Humboldt and Schouw, has persisted
in much of its primitive form to the present time, where it is represented by
innumerable lists and catalogues. Geographical distribution was grounded
upon the species, a fact which early caused it to become stereotyped as a
statistical study of little value. This tendency was emphasized by the
general practice of determining distribution for more or less artificial areas,
and of instituting comparisons between regions or continents too little
known or too widely remote. The fixed character of the subject is
conclusively shown by the fact that it still persists in almost the original
form more than a half century after Grisebach pointed out that the
formation was the real unit of vegetation, and hence of distribution.
4. The plant formation. The corner-stone of ecology was laid by
Grisebach in 1838 by his recognition of the plant formation as the
fundamental feature of vegetation. Earlier writers, notably Linné (1737,
1751), Biberg (1749), and Hedenberg (1754), had perceived this relation
more or less clearly, but failed to reduce it to a definite guiding principle.
This was a natural result of the dominance of descriptive botany in the 18th
century, by virtue of which all other lines of botanical inquiry languished.
This tendency had spent itself to a certain degree by the opening of the 19th
century, and both plant distribution and plant physiology began to take
form. The stimulus given the former by Humboldt (1807) turned the
attention of botanists more critically to the study of vegetation as a field in
itself, and the growing feeling for structure in the latter led to Grisebach’s
concept of the formation, which he defined as follows: “I would term a
group of plants which bears a definite physiognomic character, such as a
meadow, a forest, etc., a phytogeographic formation. The latter may be
3. Geographical distribution. The systematic analysis of the great field
of ecology is essential to its proper development in the future. A glance at
its history shows that, while a number of essential points of attack have
been discovered, only one or two of these have been organized, and that
there is still an almost entire lack of correlation and coordination between
these. The earliest and simplest development of the subject was concerned
with the distribution of plants. This was at first merely an offshoot of
taxonomy, and, in spite of the work of Humboldt and Schouw, has persisted
in much of its primitive form to the present time, where it is represented by
innumerable lists and catalogues. Geographical distribution was grounded
upon the species, a fact which early caused it to become stereotyped as a
statistical study of little value. This tendency was emphasized by the
general practice of determining distribution for more or less artificial areas,
and of instituting comparisons between regions or continents too little
known or too widely remote. The fixed character of the subject is
conclusively shown by the fact that it still persists in almost the original
form more than a half century after Grisebach pointed out that the
formation was the real unit of vegetation, and hence of distribution.
4. The plant formation. The corner-stone of ecology was laid by
Grisebach in 1838 by his recognition of the plant formation as the
fundamental feature of vegetation. Earlier writers, notably Linné (1737,
1751), Biberg (1749), and Hedenberg (1754), had perceived this relation
more or less clearly, but failed to reduce it to a definite guiding principle.
This was a natural result of the dominance of descriptive botany in the 18th
century, by virtue of which all other lines of botanical inquiry languished.
This tendency had spent itself to a certain degree by the opening of the 19th
century, and both plant distribution and plant physiology began to take
form. The stimulus given the former by Humboldt (1807) turned the
attention of botanists more critically to the study of vegetation as a field in
itself, and the growing feeling for structure in the latter led to Grisebach’s
concept of the formation, which he defined as follows: “I would term a
group of plants which bears a definite physiognomic character, such as a
meadow, a forest, etc., a phytogeographic formation. The latter may be
Page 27
characterized by a single social species, by a complex of dominant species
belonging to one family, or, finally, it may show an aggregate of species,
which, though of various taxonomic character, have a common peculiarity;
thus, the alpine meadows consist almost exclusively of perennial herbs.”
The acceptance of the formation as the unit of vegetation took place slowly,
but as a result of the work of Kerner (1863), Grisebach (1872), Engler
(1879), Hult (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), and
Warming (1889), this point of view came to be more and more prevalent. It
was not, however, until the appearance of three works of great importance,
Warming (1895), Drude (1896), and Schimper (1898), that the concept of
the formation became generally predominant. With the growing recognition
of the formation during the last decade has appeared the inevitable tendency
to stereotype the subject of ecology in this stage. The present need, in
consequence, is to show very clearly that the idea of the formation is a
fundamental, and not an ultimate one, and that the proper superstructure of
ecology is yet to be reared upon this as the foundation.
5. Plant succession. The fact that formations arise and disappear was
perceived by Biberg as early as 1749, but it received slight attention until
Steenstrup’s study of the succession in the forests of Zealand (1844 prox.).
In the development of formations, as well as in their recognition, nearly all
workers have confined themselves to the investigation of particular
changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff
(1864), Hult (1881), Senft (1888), Warming (1890), and others have added
much to our detailed knowledge of formational development.
Notwithstanding the lapse of more than a half century, the study of plant
successions is by no means a general practice among ecologists. This is a
ready explanation of the fact that the vast field has so far yielded but few
generalizations. Warming (1895) was the first to compile the few general
principles of development clearly indicated up to this time. The first critical
attempt to systematize the investigation of succession was made by
Clements (1904), though this can be considered as little more than a
beginning on account of the small number of successions so far studied.
Future progress in this field will be conditioned not only by the more
frequent study of developmental problems by working ecologists, but also,
and most especially, by the application of known principles of succession,
and by the working out of new ones.
belonging to one family, or, finally, it may show an aggregate of species,
which, though of various taxonomic character, have a common peculiarity;
thus, the alpine meadows consist almost exclusively of perennial herbs.”
The acceptance of the formation as the unit of vegetation took place slowly,
but as a result of the work of Kerner (1863), Grisebach (1872), Engler
(1879), Hult (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), and
Warming (1889), this point of view came to be more and more prevalent. It
was not, however, until the appearance of three works of great importance,
Warming (1895), Drude (1896), and Schimper (1898), that the concept of
the formation became generally predominant. With the growing recognition
of the formation during the last decade has appeared the inevitable tendency
to stereotype the subject of ecology in this stage. The present need, in
consequence, is to show very clearly that the idea of the formation is a
fundamental, and not an ultimate one, and that the proper superstructure of
ecology is yet to be reared upon this as the foundation.
5. Plant succession. The fact that formations arise and disappear was
perceived by Biberg as early as 1749, but it received slight attention until
Steenstrup’s study of the succession in the forests of Zealand (1844 prox.).
In the development of formations, as well as in their recognition, nearly all
workers have confined themselves to the investigation of particular
changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff
(1864), Hult (1881), Senft (1888), Warming (1890), and others have added
much to our detailed knowledge of formational development.
Notwithstanding the lapse of more than a half century, the study of plant
successions is by no means a general practice among ecologists. This is a
ready explanation of the fact that the vast field has so far yielded but few
generalizations. Warming (1895) was the first to compile the few general
principles of development clearly indicated up to this time. The first critical
attempt to systematize the investigation of succession was made by
Clements (1904), though this can be considered as little more than a
beginning on account of the small number of successions so far studied.
Future progress in this field will be conditioned not only by the more
frequent study of developmental problems by working ecologists, but also,
and most especially, by the application of known principles of succession,
and by the working out of new ones.
Page 28
6. Ecological phytogeography. Until recent years, the almost universal
tendency was to give attention to formations from the standpoint of
vegetation alone. While the habitat was touched here and there by isolated
workers, and plant functions were being studied intensively by
physiologists, both were practically ignored by ecologists as a class. The
appearance of Warming’s Lehrbuch der oecologischen Pflanzengeographie
(1896) and of Schimper’s Pflanzengeographie auf physiologischer
Grundlage (1898) remedied this condition in a measure by a general
discussion of the habitat, and by emphasizing the importance of the
ecological or physiological point of view. Despite their frank recognition of
the unique value of the habitat, the major part of both books was necessarily
given to what may be termed the general description of formations. For this
reason, and for others arising out of an almost complete dearth of methods
of investigation, ecology is still almost entirely a floristic study in practice,
although there is a universal recognition of the much greater value of the
viewpoint which rests upon the relation between the formation and its
habitat.
7. Experimental ecology. Properly speaking, the experimental study of
ecology dates from Bonnier[1] (1890, 1895), though it is well understood
that experimental adjustment of plants to certain physical factors had been
the subject of investigation before this time. The chief merit of Bonnier’s
work, however, lies in the fact that it was done out of doors, under natural
conditions, and for these reasons it should be regarded as the real beginning
of this subject. Bonnier’s experiments were made for the purpose of
determining the effect of altitude. Culture plots of certain species were
located in the Alps and the Pyrenees, and the results were compared with
control cultures made in the lowlands about Paris. In 1894 he also made a
comparative study of certain polydemic species common to the arctic
islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these methods
are fundamental to field experiment, but the results are inconclusive,
inasmuch as altitude is a complex of factors. As no careful study was made
of the latter, it was manifestly impossible to refer changes and differences
of structure to the definite cause. In a paper that has just appeared, E. S.
Clements (1905) has applied the method of polydemic comparison to nearly
a hundred species of the Rocky mountains. In this work, the all-important
advance has been made of determining accurately the decisive differences
between the two or more habitats of the same species in terms of direct
tendency was to give attention to formations from the standpoint of
vegetation alone. While the habitat was touched here and there by isolated
workers, and plant functions were being studied intensively by
physiologists, both were practically ignored by ecologists as a class. The
appearance of Warming’s Lehrbuch der oecologischen Pflanzengeographie
(1896) and of Schimper’s Pflanzengeographie auf physiologischer
Grundlage (1898) remedied this condition in a measure by a general
discussion of the habitat, and by emphasizing the importance of the
ecological or physiological point of view. Despite their frank recognition of
the unique value of the habitat, the major part of both books was necessarily
given to what may be termed the general description of formations. For this
reason, and for others arising out of an almost complete dearth of methods
of investigation, ecology is still almost entirely a floristic study in practice,
although there is a universal recognition of the much greater value of the
viewpoint which rests upon the relation between the formation and its
habitat.
7. Experimental ecology. Properly speaking, the experimental study of
ecology dates from Bonnier[1] (1890, 1895), though it is well understood
that experimental adjustment of plants to certain physical factors had been
the subject of investigation before this time. The chief merit of Bonnier’s
work, however, lies in the fact that it was done out of doors, under natural
conditions, and for these reasons it should be regarded as the real beginning
of this subject. Bonnier’s experiments were made for the purpose of
determining the effect of altitude. Culture plots of certain species were
located in the Alps and the Pyrenees, and the results were compared with
control cultures made in the lowlands about Paris. In 1894 he also made a
comparative study of certain polydemic species common to the arctic
islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these methods
are fundamental to field experiment, but the results are inconclusive,
inasmuch as altitude is a complex of factors. As no careful study was made
of the latter, it was manifestly impossible to refer changes and differences
of structure to the definite cause. In a paper that has just appeared, E. S.
Clements (1905) has applied the method of polydemic comparison to nearly
a hundred species of the Rocky mountains. In this work, the all-important
advance has been made of determining accurately the decisive differences
between the two or more habitats of the same species in terms of direct
Page 29
factors, water-content, humidity, and light. In his own investigations of
Colorado mountain vegetation, the author has applied the method of field
cultures by planting seeds of somewhat plastic species in habitats of
measured value, and has thought to initiate a new line of research by
applying experimental methods to the study of vegetation as an organism.
In connection with this, there has also been developed a method of control
experiment in the plant house under definitely measured differences of
water and light.
8. Ecology of the habitat. Since the time of Humboldt, there have been
desultory attempts to determine the physical factors of habitats with some
degree of accuracy. The first real achievement in this line was in the
measurement of light values by Wiesner in 1896. In 1898 the writer first
began to study the structure of habitats by the determination of water-
content, light, humidity, temperature, wind, etc., by means of instruments.
These methods were used by one of his pupils, Thornber (1901), in the
study of a particular formation, and by another, Hedgcock (1902), in a
critical investigation of water-content. Two years later, similar methods of
measuring physical factors were put into operation in connection with
experimental evolution under control in the plant house. E. S. Clements
(1905), as already indicated, has made the use of factor instruments the
foundation of a detailed study of polydemic species, i. e., those which grow
in two or more habitats, and which are, indeed, the most perfect of all
experiments in the production of new forms. In a volume in preparation
upon the mountain vegetation of Colorado, the writer has brought the use of
physical factor instruments to a logical conclusion, and has made the study
of the habitat the basis of the whole work. Out of this investigation has
come a new concept of vegetation (Clements 1904), namely, that it is to be
regarded as a complex organism with structures and with functions
susceptible of exact methods of study.
9. The evidence from historical development. This extremely brief
resume of what has been accomplished in the several lines of ecological
research makes evident the almost complete absence of correlation and of
system. The whole field not merely lacks system, but it also demands a
much keener perception of the relative value of the different tendencies
already developed. It is inevitable from the great number of tyros, and of
dilettante students of ecology in comparison with the few specialists, that
the surface of the field should have received all of the attention. It is,
Colorado mountain vegetation, the author has applied the method of field
cultures by planting seeds of somewhat plastic species in habitats of
measured value, and has thought to initiate a new line of research by
applying experimental methods to the study of vegetation as an organism.
In connection with this, there has also been developed a method of control
experiment in the plant house under definitely measured differences of
water and light.
8. Ecology of the habitat. Since the time of Humboldt, there have been
desultory attempts to determine the physical factors of habitats with some
degree of accuracy. The first real achievement in this line was in the
measurement of light values by Wiesner in 1896. In 1898 the writer first
began to study the structure of habitats by the determination of water-
content, light, humidity, temperature, wind, etc., by means of instruments.
These methods were used by one of his pupils, Thornber (1901), in the
study of a particular formation, and by another, Hedgcock (1902), in a
critical investigation of water-content. Two years later, similar methods of
measuring physical factors were put into operation in connection with
experimental evolution under control in the plant house. E. S. Clements
(1905), as already indicated, has made the use of factor instruments the
foundation of a detailed study of polydemic species, i. e., those which grow
in two or more habitats, and which are, indeed, the most perfect of all
experiments in the production of new forms. In a volume in preparation
upon the mountain vegetation of Colorado, the writer has brought the use of
physical factor instruments to a logical conclusion, and has made the study
of the habitat the basis of the whole work. Out of this investigation has
come a new concept of vegetation (Clements 1904), namely, that it is to be
regarded as a complex organism with structures and with functions
susceptible of exact methods of study.
9. The evidence from historical development. This extremely brief
resume of what has been accomplished in the several lines of ecological
research makes evident the almost complete absence of correlation and of
system. The whole field not merely lacks system, but it also demands a
much keener perception of the relative value of the different tendencies
already developed. It is inevitable from the great number of tyros, and of
dilettante students of ecology in comparison with the few specialists, that
the surface of the field should have received all of the attention. It is,
Page 30
however, both unfortunate and unscientific that great lines of development
should be entirely unknown to all but a few. There is no other department of
botany in which the superficial study of more than half a century ago still
prevails to the exclusion of better methods, many of which have been
known for a decade or more. It is clear, then, that the imperative need of
ecology is the proper coordination of its various points of view, and the
working out of a definite system which will make possible a ready
recognition of that which is fundamental and of that which is merely
collateral. The historical development, as is well understood, can furnish
but a slight clue to this. It is a fact of common knowledge that the first
development of any subject is general, and usually superficial also. True
values come out clearly only after the whole field has been surveyed. For
these reasons, as will be pointed out in detail later, the newer viewpoints are
regarded as either the most important or the most fundamental.
Experimental ecology will throw a flood of light upon plant structure and
function, while exact methods of studying the habitat are practically certain
of universal application in the future.
should be entirely unknown to all but a few. There is no other department of
botany in which the superficial study of more than half a century ago still
prevails to the exclusion of better methods, many of which have been
known for a decade or more. It is clear, then, that the imperative need of
ecology is the proper coordination of its various points of view, and the
working out of a definite system which will make possible a ready
recognition of that which is fundamental and of that which is merely
collateral. The historical development, as is well understood, can furnish
but a slight clue to this. It is a fact of common knowledge that the first
development of any subject is general, and usually superficial also. True
values come out clearly only after the whole field has been surveyed. For
these reasons, as will be pointed out in detail later, the newer viewpoints are
regarded as either the most important or the most fundamental.
Experimental ecology will throw a flood of light upon plant structure and
function, while exact methods of studying the habitat are practically certain
of universal application in the future.
Page 31
PRESENT STATUS OF ECOLOGY
10. The lack of special training. The bane of the recent development
popularly known as ecology has been a widespread feeling that anyone can
do ecological work, regardless of preparation. There is nothing in modern
botany more erroneous than this feeling. The whole task of ecology is to
find out what the living plant and the living formation are doing and have
done in response to definite complexes of factors, i. e., habitats. In this
sense, ecology is practically coextensive with botany, and the student of a
local flora who knows a few hundred species is no more competent to do
ecological work than he is to reconstruct the phylogeny of the vegetable
kingdom, or to explain the transmission of ancestral qualities. The
comprehensive and fundamental character of the subject makes a broad
special training even more requisite than in more restricted lines of
botanical inquiry. The ecologist must first of all be a botanist, not a mere
cataloguer of plants, and he must also possess a particular training in the
special methods of ecological research. He must be familiar with the
various points of attack in this field, and he must know the history of his
subject thoroughly. Ecology affords the most striking example of the
prevalent evil of American botanical study, i. e., an indifference to, or an
ignorance of the literature of the subject. The trouble is much aggravated
here, however, by the breadth of the field, and the common assumption that
a special training is unnecessary, if not, indeed, superfluous. Ignorance of
the important ecological literature has been a most fertile source of crude
and superficial studies, a condition that will become more apparent as these
fields are worked again by carefully trained investigators.
11. Descriptive ecology. The stage of development of the subject at the
present time may be designated as descriptive ecology, for purposes of
discussion merely. This is concerned with the superficial description of
vegetation in general terms, and results from the fact that the development
has begun on the surface, and has scarcely penetrated beneath it. The
organic connection between ecology and floristic has produced an
erroneous impression as to the relative value of the two. Floristic has
required little knowledge, and less preparation: it lends itself with insidious
ease to chance journeys or to vacation trips, the fruits of which are found in
10. The lack of special training. The bane of the recent development
popularly known as ecology has been a widespread feeling that anyone can
do ecological work, regardless of preparation. There is nothing in modern
botany more erroneous than this feeling. The whole task of ecology is to
find out what the living plant and the living formation are doing and have
done in response to definite complexes of factors, i. e., habitats. In this
sense, ecology is practically coextensive with botany, and the student of a
local flora who knows a few hundred species is no more competent to do
ecological work than he is to reconstruct the phylogeny of the vegetable
kingdom, or to explain the transmission of ancestral qualities. The
comprehensive and fundamental character of the subject makes a broad
special training even more requisite than in more restricted lines of
botanical inquiry. The ecologist must first of all be a botanist, not a mere
cataloguer of plants, and he must also possess a particular training in the
special methods of ecological research. He must be familiar with the
various points of attack in this field, and he must know the history of his
subject thoroughly. Ecology affords the most striking example of the
prevalent evil of American botanical study, i. e., an indifference to, or an
ignorance of the literature of the subject. The trouble is much aggravated
here, however, by the breadth of the field, and the common assumption that
a special training is unnecessary, if not, indeed, superfluous. Ignorance of
the important ecological literature has been a most fertile source of crude
and superficial studies, a condition that will become more apparent as these
fields are worked again by carefully trained investigators.
11. Descriptive ecology. The stage of development of the subject at the
present time may be designated as descriptive ecology, for purposes of
discussion merely. This is concerned with the superficial description of
vegetation in general terms, and results from the fact that the development
has begun on the surface, and has scarcely penetrated beneath it. The
organic connection between ecology and floristic has produced an
erroneous impression as to the relative value of the two. Floristic has
required little knowledge, and less preparation: it lends itself with insidious
ease to chance journeys or to vacation trips, the fruits of which are found in
Page 32
vague descriptive articles, and in the multiplication of fictitious formations.
While there is good reason that a record should be left of any serious
reconnaissance, even though it be of a few weeks’ duration, the resulting
lists and descriptive articles can have only the most rudimentary value, and
it is absurd to regard them as ecological contributions at all. No statement
admits of stronger emphasis, and there is none that should be taken more
closely to heart by botanists who have supposed that they were doing
ecological work. An almost equally fertile source of valueless work is the
independent treatment of a restricted local area. The great readiness with
which floristic lists and descriptions can be made has led many a botanist,
working in a small area, or passing hurriedly through an extended region, to
try his hand at formation making. From this practice have resulted scores of
so-called formations, which are mere patches, consocies, or stages in
development, or which have, indeed, no existence other than in the minds of
their discoverers. The misleading definiteness which a photograph seems to
give a bit of vegetation has been responsible for a surplus of photographic
formations, which have no counterparts in nature. Indispensable as the
photograph is to any systematic record of vegetation, its use up to the
present time has but too often served to bring it into disrepute. There has
been a marked tendency to apply the current methods of descriptive botany
to vegetation, and to regard every slightly different piece of the floral
covering as a formation. No method can yield results further from the truth.
It is evident that the recognition and limitation of formations should be left
absolutely to the broadly trained specialist, who has a thorough preparation
by virtue of having acquainted himself carefully with the development and
structure of typical formations over large areas.
12. The value of floristic. In what has been said above, there is no intent
to decry the value of floristic. The skilled workman can spare the material
which he is fashioning as readily as the ecologist can work without an
accurate knowledge of the genera and species which make up a particular
vegetation. Some botanists whose knowledge of ecology is that of the study
or the laboratory have maintained that it is possible to investigate vegetation
without knowing the plants which compose it. Ecology is to be wrought out
in the field, however, and the field ecologist—none other, indeed, should
bear the name—understands that floristic alone can furnish the crude
material which takes form under his hands. It is the absolute need of a
thorough acquaintance with the flora of a region which makes it impossible
While there is good reason that a record should be left of any serious
reconnaissance, even though it be of a few weeks’ duration, the resulting
lists and descriptive articles can have only the most rudimentary value, and
it is absurd to regard them as ecological contributions at all. No statement
admits of stronger emphasis, and there is none that should be taken more
closely to heart by botanists who have supposed that they were doing
ecological work. An almost equally fertile source of valueless work is the
independent treatment of a restricted local area. The great readiness with
which floristic lists and descriptions can be made has led many a botanist,
working in a small area, or passing hurriedly through an extended region, to
try his hand at formation making. From this practice have resulted scores of
so-called formations, which are mere patches, consocies, or stages in
development, or which have, indeed, no existence other than in the minds of
their discoverers. The misleading definiteness which a photograph seems to
give a bit of vegetation has been responsible for a surplus of photographic
formations, which have no counterparts in nature. Indispensable as the
photograph is to any systematic record of vegetation, its use up to the
present time has but too often served to bring it into disrepute. There has
been a marked tendency to apply the current methods of descriptive botany
to vegetation, and to regard every slightly different piece of the floral
covering as a formation. No method can yield results further from the truth.
It is evident that the recognition and limitation of formations should be left
absolutely to the broadly trained specialist, who has a thorough preparation
by virtue of having acquainted himself carefully with the development and
structure of typical formations over large areas.
12. The value of floristic. In what has been said above, there is no intent
to decry the value of floristic. The skilled workman can spare the material
which he is fashioning as readily as the ecologist can work without an
accurate knowledge of the genera and species which make up a particular
vegetation. Some botanists whose knowledge of ecology is that of the study
or the laboratory have maintained that it is possible to investigate vegetation
without knowing the plants which compose it. Ecology is to be wrought out
in the field, however, and the field ecologist—none other, indeed, should
bear the name—understands that floristic alone can furnish the crude
material which takes form under his hands. It is the absolute need of a
thorough acquaintance with the flora of a region which makes it impossible
Page 33
for a traveler to obtain anything of real ecological value in his first journey
through a country. As the very first step, he must gain at least a fair
knowledge of the floristic, which will alone take the major part of one or
more growing seasons. This information the student of a local flora already
has at the tip of his tongue; in itself it can not constitute a contribution to
ecology, but merely the basis for one. In this connection, moreover, it can
not be used independently, but becomes of value only after an acquaintance
with a wide field. Floristic study and floristic lists, then, are indispensable,
but to be of real value their proper function must be clearly recognized.
They do not constitute ecology.
13. Reconnaissance and investigation. In striving to indicate the true
value and worth of ecological study, it becomes necessary to draw a definite
line between what we may term reconnaissance and investigation. By the
former is understood the preliminary survey of a region, extending over one
or two years. The objects of such a survey are to obtain a comprehensive
general knowledge of the topography and vegetation of the region, and of
its relation to the other regions about it. The chief purpose, however, is to
gain a good working acquaintance with the flora: a reconnaissance to be of
value must do this at all events. Certain general facts will inevitably appear
during this process, but they will invariably need the confirmation of future
study. It would be an advantage to real ecology if reconnaissance were to
confine itself entirely to the matter of making a careful floristic survey.
Investigation begins when the inquiry is directed to the habitat, or to the
development and structure of the formation which it bears, i. e., when it
takes up the manifold problems of the οἶκος. Such a study must be based
upon floristic, but the latter becomes a part of investigation only in so far as
it leads to it. Standing by itself, it is not ecological research: it is the
preparation for it. This distinction deserves careful thought. The numerous
recruits to ecology have turned their attention to what lay nearest to hand,
with little question as to its value, or to where it might lead. The result has
been to make reconnaissance far outweigh investigation in amount, and to
give it a value which properly belongs to the latter. Furthermore, this
mistaken conception has in many cases, without doubt, prevented its
leading to valuable research work.
14. Resident investigation. Obviously, if reconnaissance is a superficial
survey, and investigation thorough extensive study, an important distinction
between them is in the time required. While one may well be the result of a
through a country. As the very first step, he must gain at least a fair
knowledge of the floristic, which will alone take the major part of one or
more growing seasons. This information the student of a local flora already
has at the tip of his tongue; in itself it can not constitute a contribution to
ecology, but merely the basis for one. In this connection, moreover, it can
not be used independently, but becomes of value only after an acquaintance
with a wide field. Floristic study and floristic lists, then, are indispensable,
but to be of real value their proper function must be clearly recognized.
They do not constitute ecology.
13. Reconnaissance and investigation. In striving to indicate the true
value and worth of ecological study, it becomes necessary to draw a definite
line between what we may term reconnaissance and investigation. By the
former is understood the preliminary survey of a region, extending over one
or two years. The objects of such a survey are to obtain a comprehensive
general knowledge of the topography and vegetation of the region, and of
its relation to the other regions about it. The chief purpose, however, is to
gain a good working acquaintance with the flora: a reconnaissance to be of
value must do this at all events. Certain general facts will inevitably appear
during this process, but they will invariably need the confirmation of future
study. It would be an advantage to real ecology if reconnaissance were to
confine itself entirely to the matter of making a careful floristic survey.
Investigation begins when the inquiry is directed to the habitat, or to the
development and structure of the formation which it bears, i. e., when it
takes up the manifold problems of the οἶκος. Such a study must be based
upon floristic, but the latter becomes a part of investigation only in so far as
it leads to it. Standing by itself, it is not ecological research: it is the
preparation for it. This distinction deserves careful thought. The numerous
recruits to ecology have turned their attention to what lay nearest to hand,
with little question as to its value, or to where it might lead. The result has
been to make reconnaissance far outweigh investigation in amount, and to
give it a value which properly belongs to the latter. Furthermore, this
mistaken conception has in many cases, without doubt, prevented its
leading to valuable research work.
14. Resident investigation. Obviously, if reconnaissance is a superficial
survey, and investigation thorough extensive study, an important distinction
between them is in the time required. While one may well be the result of a
Page 34
journey of some duration, the other is essentially dependent upon residence.
In the past the great disparity between the size of the field and the number
of workers has made resident study too often an ideal, but in the future it
will be increasingly the case that a particular region will be worked by a
trained ecologist resident in it. This may never be altogether true of
inaccessible and sterile portions of the globe. It may be pointed out,
however, that, between the tropics and the poles, residence during the
summer or growing period is in essence continuous residence. In the
ultimate analysis, winter conditions have of course some influence upon the
development of vegetation during the summer, but the important problems
which a vegetation presents must be worked out during the period of
development. For temperate, arctic, and alpine regions, then, repeated study
during the growing period for a term of years has practically all the
advantages of continuous residence. For all practical purposes, it is resident
study.
15. The dangers of a restricted field. In the resident study of a
particular region, the temptation to make an intensive investigation of a
circumscribed area is very strong. The limits imposed by distance are alone
a sufficient explanation of this, but it is greatly increased by the inclination
toward detailed study for which a small field offers opportunity. This
temptation can be overcome only by a general preliminary study of the
larger region in which the particular field is located. The broader outlook
gained in this way will throw needed light upon many obscure facts of the
latter, and at the same time it will act as a necessary corrective of the
tendency to consider the problems of the local field in a detached manner,
and to magnify the value of the distinctions made and the results obtained.
Such a general survey has the purpose and value of a reconnaissance, and is
always the first step in the accurate and detailed investigation of the local
area or formation. Each corrects the extreme tendency of the other, and
thorough comprehensive work can be done only by combining the two
methods. When the field of inquiry is a large area or covers a whole region,
the procedure should be essentially the same. A third stage must be added,
however, in which a more careful survey is made of the entire field in the
light of the thorough study of the local area. The writer’s methods in the
investigation of the Colorado vegetation illustrate this procedure. The
summers of 1896, 1897, 1898 were devoted to reconnaissance; those of
1899–1904 were given to detailed and comprehensive study by instrument
In the past the great disparity between the size of the field and the number
of workers has made resident study too often an ideal, but in the future it
will be increasingly the case that a particular region will be worked by a
trained ecologist resident in it. This may never be altogether true of
inaccessible and sterile portions of the globe. It may be pointed out,
however, that, between the tropics and the poles, residence during the
summer or growing period is in essence continuous residence. In the
ultimate analysis, winter conditions have of course some influence upon the
development of vegetation during the summer, but the important problems
which a vegetation presents must be worked out during the period of
development. For temperate, arctic, and alpine regions, then, repeated study
during the growing period for a term of years has practically all the
advantages of continuous residence. For all practical purposes, it is resident
study.
15. The dangers of a restricted field. In the resident study of a
particular region, the temptation to make an intensive investigation of a
circumscribed area is very strong. The limits imposed by distance are alone
a sufficient explanation of this, but it is greatly increased by the inclination
toward detailed study for which a small field offers opportunity. This
temptation can be overcome only by a general preliminary study of the
larger region in which the particular field is located. The broader outlook
gained in this way will throw needed light upon many obscure facts of the
latter, and at the same time it will act as a necessary corrective of the
tendency to consider the problems of the local field in a detached manner,
and to magnify the value of the distinctions made and the results obtained.
Such a general survey has the purpose and value of a reconnaissance, and is
always the first step in the accurate and detailed investigation of the local
area or formation. Each corrects the extreme tendency of the other, and
thorough comprehensive work can be done only by combining the two
methods. When the field of inquiry is a large area or covers a whole region,
the procedure should be essentially the same. A third stage must be added,
however, in which a more careful survey is made of the entire field in the
light of the thorough study of the local area. The writer’s methods in the
investigation of the Colorado vegetation illustrate this procedure. The
summers of 1896, 1897, 1898 were devoted to reconnaissance; those of
1899–1904 were given to detailed and comprehensive study by instrument
Page 35
and quadrat of a highly diversified, representative area less than 20 miles
square, while the work of the final summer will be the application of the
results obtained in this localized area to the region traversed from 1896–98.
This is practically the application of methods of precision to an area of
more than 100,000 square miles. It also serves to call attention to another
point not properly appreciated as yet by those who would do ecological
work. This is the need of taking up field problems as a result of serious
forethought, and not as a matter of accident or mere propinquity. A carefully
matured plan of attack which contemplates an expenditure of time and
energy for a number of years will yield results of value, no matter how
much attention an area may have received. On the other hand, an aimless or
hurried excursion into the least known or richest of regions will lead to
nothing but a waste of time, especially upon the part of the ecologist, who
must read the articles which result, if only for the purpose of making sure
that there is nothing in them.
square, while the work of the final summer will be the application of the
results obtained in this localized area to the region traversed from 1896–98.
This is practically the application of methods of precision to an area of
more than 100,000 square miles. It also serves to call attention to another
point not properly appreciated as yet by those who would do ecological
work. This is the need of taking up field problems as a result of serious
forethought, and not as a matter of accident or mere propinquity. A carefully
matured plan of attack which contemplates an expenditure of time and
energy for a number of years will yield results of value, no matter how
much attention an area may have received. On the other hand, an aimless or
hurried excursion into the least known or richest of regions will lead to
nothing but a waste of time, especially upon the part of the ecologist, who
must read the articles which result, if only for the purpose of making sure
that there is nothing in them.
Page 36
APPLICATIONS OF ECOLOGY
16. The subjects touched by ecology. The applications of ecological
methods and results to other departments of botany, and to other fields of
research are numerous. Many of these are both intimate and fundamental,
and give promise of affording new and extremely fruitful points of view. It
has already been indicated that ecology bears the closest of relations to
morphology and histology on the one side, and to physiology on the other—
that it is, indeed, nothing but a rational field physiology, which regards form
and function as inseparable phenomena. The closeness with which it
touches plant pathology follows directly from this, as pathology is nothing
more than abnormal form and functioning. Experimental work in ecology is
purely a study of evolution, and the facts of the latter are the materials with
which taxonomy deals. Forestry has already been termed “applied ecology”
and in its scientific aspects, which are its foundation, it is precisely the
ecology of woody plants, and of the vegetation which they constitute. Apart
from botany, the physical side of ecology is largely a question of soil
physics, and of physiography. On the other hand, vegetation is coming more
and more to be regarded as a fundamental factor in zoogeography and in
sociology. Furthermore, with respect to the latter, it will be pointed out
below that the principles of association which have been determined for
plants, viz., invasion, succession, zonation, and alternation, apply with
almost equal force to man.
17. Physiology and pathology. The effect of ecology in emphasizing the
intrinsically close connection between physiology and morphology has
already been mentioned. Its influence in normalizing the former by forcing
it into the field as the place for experiment, and by directing the chief
attention to the plant as an organism rather than a complex of organs, is also
rapidly coming to be felt. Ecology will doubtless exert a corrective
influence upon pathology in the near future. This is inevitable as the latter
ceases to be the merely formal study of specific pathogenic organisms, and
turns its attention to the cause of all abnormality, which is to be found in the
habitat, whether this be physical, as when the water-content is low, or
biotic, when a parasitic fungus is present. The relative ease with which
specific diseases can be studied has helped to obscure the essential fact that
16. The subjects touched by ecology. The applications of ecological
methods and results to other departments of botany, and to other fields of
research are numerous. Many of these are both intimate and fundamental,
and give promise of affording new and extremely fruitful points of view. It
has already been indicated that ecology bears the closest of relations to
morphology and histology on the one side, and to physiology on the other—
that it is, indeed, nothing but a rational field physiology, which regards form
and function as inseparable phenomena. The closeness with which it
touches plant pathology follows directly from this, as pathology is nothing
more than abnormal form and functioning. Experimental work in ecology is
purely a study of evolution, and the facts of the latter are the materials with
which taxonomy deals. Forestry has already been termed “applied ecology”
and in its scientific aspects, which are its foundation, it is precisely the
ecology of woody plants, and of the vegetation which they constitute. Apart
from botany, the physical side of ecology is largely a question of soil
physics, and of physiography. On the other hand, vegetation is coming more
and more to be regarded as a fundamental factor in zoogeography and in
sociology. Furthermore, with respect to the latter, it will be pointed out
below that the principles of association which have been determined for
plants, viz., invasion, succession, zonation, and alternation, apply with
almost equal force to man.
17. Physiology and pathology. The effect of ecology in emphasizing the
intrinsically close connection between physiology and morphology has
already been mentioned. Its influence in normalizing the former by forcing
it into the field as the place for experiment, and by directing the chief
attention to the plant as an organism rather than a complex of organs, is also
rapidly coming to be felt. Ecology will doubtless exert a corrective
influence upon pathology in the near future. This is inevitable as the latter
ceases to be the merely formal study of specific pathogenic organisms, and
turns its attention to the cause of all abnormality, which is to be found in the
habitat, whether this be physical, as when the water-content is low, or
biotic, when a parasitic fungus is present. The relative ease with which
specific diseases can be studied has helped to obscure the essential fact that
Page 37
the approach to pathology must be through physiology. Much indeed of the
observational physiology of the laboratories has been pathology, and it will
be impossible to draw a clear line between them until precise experiment in
the habitat has come into vogue.
18. Experimental evolution. As a result of the extremely fragmentary
character of the geological record, nothing is more absolute than that there
can be no positive knowledge of the exact origin of a form or species,
except in those rare cases of the present day, where the whole process has
taken place under the eye of a trained observer. The origin of the plant
forms known at present must forever lie without the domain of direct
knowledge. If it were possible, by a marvel of ingenuity and patience to
develop experimentally Myosurus from Selaginella, this would not be
absolutely conclusive proof that Myosurus was first derived in this way.
When all is said, however, this would be the very best of presumptive
evidence. It must also be recognized that this is the nearest to complete
proof that we shall ever attain, and with this in mind it becomes apparent at
once that evidence from experiment is of paramount importance in the
study of evolution (the origin of species).
The phase of experimental ecology which has to do with the plant has
well been called experimental evolution. While this is a field almost wholly
without development at present, there can be little question that it is to be
one of the most fertile and important in the future. Attention will be directed
first to those forms which are undergoing modification at the present time.
The cause and direction of change will be ascertained, and its amount and
rapidity measured by biometrical methods. The next step will be to actually
change the habitat of representative types, and to determine for each the
general trend of adaptation, as well as the exact details. By means of the
methods used and the results obtained in these investigations, it will be
possible to attack the much more difficult problem of retracing the
development of species already definitely constituted. This will be
accomplished by the study of the derived and the supposed ancestral form,
but owing to the great preponderance of evolution over reversion, the study
of the ancestral form will yield much more valuable results.
The general application of the methods of experimental ecology will
mark a new era in the study of evolution. There has been a surplus of
literary investigation, but altogether too little actual experiment. The great
observational physiology of the laboratories has been pathology, and it will
be impossible to draw a clear line between them until precise experiment in
the habitat has come into vogue.
18. Experimental evolution. As a result of the extremely fragmentary
character of the geological record, nothing is more absolute than that there
can be no positive knowledge of the exact origin of a form or species,
except in those rare cases of the present day, where the whole process has
taken place under the eye of a trained observer. The origin of the plant
forms known at present must forever lie without the domain of direct
knowledge. If it were possible, by a marvel of ingenuity and patience to
develop experimentally Myosurus from Selaginella, this would not be
absolutely conclusive proof that Myosurus was first derived in this way.
When all is said, however, this would be the very best of presumptive
evidence. It must also be recognized that this is the nearest to complete
proof that we shall ever attain, and with this in mind it becomes apparent at
once that evidence from experiment is of paramount importance in the
study of evolution (the origin of species).
The phase of experimental ecology which has to do with the plant has
well been called experimental evolution. While this is a field almost wholly
without development at present, there can be little question that it is to be
one of the most fertile and important in the future. Attention will be directed
first to those forms which are undergoing modification at the present time.
The cause and direction of change will be ascertained, and its amount and
rapidity measured by biometrical methods. The next step will be to actually
change the habitat of representative types, and to determine for each the
general trend of adaptation, as well as the exact details. By means of the
methods used and the results obtained in these investigations, it will be
possible to attack the much more difficult problem of retracing the
development of species already definitely constituted. This will be
accomplished by the study of the derived and the supposed ancestral form,
but owing to the great preponderance of evolution over reversion, the study
of the ancestral form will yield much more valuable results.
The general application of the methods of experimental ecology will
mark a new era in the study of evolution. There has been a surplus of
literary investigation, but altogether too little actual experiment. The great
Page 38
value of De Vries’ work lies not in the importance of the results obtained,
but in calling attention to the unique importance of experimental methods in
contributing to a knowledge of evolution. The development of the latter has
been greatly hindered by the dearth of actual facts, and by a marked
tendency to compensate for this by verbiage and dogmatism. This is well
illustrated by the present position of the “mutation theory,” which, so far as
the evidence available is concerned, is merely a working hypothesis. An
incredible amount of bias and looseness of thought have characterized the
discussion of evolution. It is earnestly to be hoped that the future will bring
more work and less argument, and that the literary evolutionists will
become less and less reluctant to leave the relative merits of variation and
mutation to experiment.
19. Taxonomy. Taxonomy is classified evolution. It is distinct from
descriptive botany, which is merely a cataloguing of all known forms, with
little regard to development and relationship. The consideration of the latter
is peculiarly the problem of taxonomy, but the solution must be sought
through experimental evolution. The first task of the latter is to determine
the course of modification in related forms, and the relationships existing
between them. With this information, taxonomy can group forms according
to their rank, i. e., their descent. The same method is applicable to the
species of a genus, and, in a less degree, perhaps, to the genera which
constitute a family. The use to which it may be put in indicating family
relationships will depend largely upon the gap existing between the families
concerned. While interpretation will always play a part in taxonomy, the
general use of experiment will leave much less opportunity for the personal
equation than is at present the case. Taxonomy, like descriptive botany, is
based upon the species, but, while there may exist a passable kind of
descriptive botany, there can be no real taxonomy as long as the sole
criterion of a species is the difference which any observer thinks he sees
between one plant and another. The so-called species of to-day range in
value from mere variations to true species which are groups of great
constancy and definiteness. The reasons for this are obvious when one
recalls that “species” are still the product of the herbarium, not of the field,
and that the more intensive the study, the greater the output in “species.” It
would seem that careful field study of a form for several seasons would be
the first requisite for the making of a species, but it is a precaution which is
entirely ignored in the vast majority of cases. The thought of subjecting
but in calling attention to the unique importance of experimental methods in
contributing to a knowledge of evolution. The development of the latter has
been greatly hindered by the dearth of actual facts, and by a marked
tendency to compensate for this by verbiage and dogmatism. This is well
illustrated by the present position of the “mutation theory,” which, so far as
the evidence available is concerned, is merely a working hypothesis. An
incredible amount of bias and looseness of thought have characterized the
discussion of evolution. It is earnestly to be hoped that the future will bring
more work and less argument, and that the literary evolutionists will
become less and less reluctant to leave the relative merits of variation and
mutation to experiment.
19. Taxonomy. Taxonomy is classified evolution. It is distinct from
descriptive botany, which is merely a cataloguing of all known forms, with
little regard to development and relationship. The consideration of the latter
is peculiarly the problem of taxonomy, but the solution must be sought
through experimental evolution. The first task of the latter is to determine
the course of modification in related forms, and the relationships existing
between them. With this information, taxonomy can group forms according
to their rank, i. e., their descent. The same method is applicable to the
species of a genus, and, in a less degree, perhaps, to the genera which
constitute a family. The use to which it may be put in indicating family
relationships will depend largely upon the gap existing between the families
concerned. While interpretation will always play a part in taxonomy, the
general use of experiment will leave much less opportunity for the personal
equation than is at present the case. Taxonomy, like descriptive botany, is
based upon the species, but, while there may exist a passable kind of
descriptive botany, there can be no real taxonomy as long as the sole
criterion of a species is the difference which any observer thinks he sees
between one plant and another. The so-called species of to-day range in
value from mere variations to true species which are groups of great
constancy and definiteness. The reasons for this are obvious when one
recalls that “species” are still the product of the herbarium, not of the field,
and that the more intensive the study, the greater the output in “species.” It
would seem that careful field study of a form for several seasons would be
the first requisite for the making of a species, but it is a precaution which is
entirely ignored in the vast majority of cases. The thought of subjecting
Page 39
forms presumed to be species to conclusive test by experiment has
apparently not even occurred to descriptive botanists as yet.
Notwithstanding, there can be no serious doubt that the existing practice of
re-splitting hairs must come to an end sooner or later. The remedy will
come from without through the application of experimental methods in the
hands of the ecologist, and the cataloguing of slight and unrelated
differences will yield to an ordered taxonomy.
Experimental evolution will solve a taxonomic problem as yet
untouched, namely, the effect of recent environment upon the production of
species. It is well understood that some species grow in nature in various
habitats without suffering material change, while others are modified to
constitute a new form in each habitat. It is at once clear that these forms (or
ecads) are of more recent descent than the species, i. e., of lower rank. It
must also be recognized that a constant group and a highly plastic one are
essentially different. If constancy is made a necessary quality of a species,
one is a species, the other is not. If both are species, then two different kinds
must be distinguished. Among the species of our manuals are found many
ecads, alongside of constant and inconstant species. These can be
distinguished only by field experiment, and their proper coordination is
possible only after this has been done. Indeed, the whole question of the
ability or the inability of environmental variation to produce constant
species is one that must be referred to repeated and long-continued
experiment in the field.
A minor service of considerable value can be rendered taxonomy by
working over the diagnosis from the ecological standpoint. Many ecological
facts are of real diagnostic value, while others are at least of much interest,
and serve to direct attention to the plant as a living thing. The loose use of
terms denoting abundance, which prevails in lists and manuals, should be
replaced by the exact usage which the quadrat method has made possible
for vegetation. The designation of habitats could be made much more exact,
and the formation, as well as the habitat form or ecad, and the vegetation
form or phyad, should be indicated in addition. The general terms drawn
from pollination, seed-production, and dissemination might also be
included to advantage.
20. Forestry, if the purely commercial aspects be disregarded, is the
ecology of a particular kind of vegetation, the forest. Therefore, in pointing
apparently not even occurred to descriptive botanists as yet.
Notwithstanding, there can be no serious doubt that the existing practice of
re-splitting hairs must come to an end sooner or later. The remedy will
come from without through the application of experimental methods in the
hands of the ecologist, and the cataloguing of slight and unrelated
differences will yield to an ordered taxonomy.
Experimental evolution will solve a taxonomic problem as yet
untouched, namely, the effect of recent environment upon the production of
species. It is well understood that some species grow in nature in various
habitats without suffering material change, while others are modified to
constitute a new form in each habitat. It is at once clear that these forms (or
ecads) are of more recent descent than the species, i. e., of lower rank. It
must also be recognized that a constant group and a highly plastic one are
essentially different. If constancy is made a necessary quality of a species,
one is a species, the other is not. If both are species, then two different kinds
must be distinguished. Among the species of our manuals are found many
ecads, alongside of constant and inconstant species. These can be
distinguished only by field experiment, and their proper coordination is
possible only after this has been done. Indeed, the whole question of the
ability or the inability of environmental variation to produce constant
species is one that must be referred to repeated and long-continued
experiment in the field.
A minor service of considerable value can be rendered taxonomy by
working over the diagnosis from the ecological standpoint. Many ecological
facts are of real diagnostic value, while others are at least of much interest,
and serve to direct attention to the plant as a living thing. The loose use of
terms denoting abundance, which prevails in lists and manuals, should be
replaced by the exact usage which the quadrat method has made possible
for vegetation. The designation of habitats could be made much more exact,
and the formation, as well as the habitat form or ecad, and the vegetation
form or phyad, should be indicated in addition. The general terms drawn
from pollination, seed-production, and dissemination might also be
included to advantage.
20. Forestry, if the purely commercial aspects be disregarded, is the
ecology of a particular kind of vegetation, the forest. Therefore, in pointing
Page 40
out the connection between them, it is only necessary to say that whatever
contributes to the ecology of the forest is a contribution to forestry. There
are, however, certain lines of inquiry which are of fundamental importance.
First among these, and of primary interest from the practical point of view,
are the questions pertaining to the distribution of forests and their structure.
Of even greater significance are the problems of forest development,
movement, and of reforestation, which are comprised in succession. The
gradual invasion of the plains and prairies by the forest belt of the east and
north is in full conformity with the laws of invasion, and the ecological
methods to be employed here serve not merely to determine the actual
conditions at present, but also to forecast them with a great deal of
accuracy. The slow but certain development of forests on new soils, and
their more rapid reestablishment where the woody vegetation has been
destroyed by burning or lumbering, are ordinary phenomena of succession,
for which the ecologist has already worked out the laws, and determined the
methods of investigation. Having once ascertained the original and adjacent
vegetation and the character of the habitat, the ecologist can indicate with
accuracy not only the character of the new forest that will appear, but also
the nature of the antecedent formations. A full knowledge of the character
and laws of succession will prove of the greatest value to the forester in all
studies of forestation and reforestation. Forests which now seem entirely
unrelated will be seen to possess the most intimate developmental
connection, and the fuller insight into the life history gained in this way will
have a direct bearing upon methods of conservation, etc. It will further
show that the forester must know other vegetations as well, since grassland
and thicket formations have an intimate influence upon the course of the
succession, as well as upon the advance of a forest frontier.
One of the greatest aids which modern ecology can furnish forestry,
however, is the method of determining the physical nature of the habitat. So
far, foresters have been obliged to content themselves with a more or less
superficial study of the structure of forest formations, without being able to
do more than guess at the physical causes which control both structure and
development. This handicap is especially noticeable in the case of forest
plantings in non-forested regions, where it has been impossible to estimate
the chances of success, or to determine the most favorable areas except by
actual plantations. Equipped with the proper instruments for measuring
water-content, humidity, light and temperature, the ecologist is able to
contributes to the ecology of the forest is a contribution to forestry. There
are, however, certain lines of inquiry which are of fundamental importance.
First among these, and of primary interest from the practical point of view,
are the questions pertaining to the distribution of forests and their structure.
Of even greater significance are the problems of forest development,
movement, and of reforestation, which are comprised in succession. The
gradual invasion of the plains and prairies by the forest belt of the east and
north is in full conformity with the laws of invasion, and the ecological
methods to be employed here serve not merely to determine the actual
conditions at present, but also to forecast them with a great deal of
accuracy. The slow but certain development of forests on new soils, and
their more rapid reestablishment where the woody vegetation has been
destroyed by burning or lumbering, are ordinary phenomena of succession,
for which the ecologist has already worked out the laws, and determined the
methods of investigation. Having once ascertained the original and adjacent
vegetation and the character of the habitat, the ecologist can indicate with
accuracy not only the character of the new forest that will appear, but also
the nature of the antecedent formations. A full knowledge of the character
and laws of succession will prove of the greatest value to the forester in all
studies of forestation and reforestation. Forests which now seem entirely
unrelated will be seen to possess the most intimate developmental
connection, and the fuller insight into the life history gained in this way will
have a direct bearing upon methods of conservation, etc. It will further
show that the forester must know other vegetations as well, since grassland
and thicket formations have an intimate influence upon the course of the
succession, as well as upon the advance of a forest frontier.
One of the greatest aids which modern ecology can furnish forestry,
however, is the method of determining the physical nature of the habitat. So
far, foresters have been obliged to content themselves with a more or less
superficial study of the structure of forest formations, without being able to
do more than guess at the physical causes which control both structure and
development. This handicap is especially noticeable in the case of forest
plantings in non-forested regions, where it has been impossible to estimate
the chances of success, or to determine the most favorable areas except by
actual plantations. Equipped with the proper instruments for measuring
water-content, humidity, light and temperature, the ecologist is able to
Page 41
determine the precise conditions under which reproduction is occurring, and
to ascertain what non-forested areas offer the most nearly similar
conditions. A knowledge of habitats and the means of measuring them
enables the forester to discover the causes which control the vegetation with
which he is already familiar, and to forecast results otherwise hidden.
Furthermore, it makes it possible for him to enter a new region and to
determine its nature and capabilities at a minimum of time and energy.
21. Physiography. Physiographic features play an important part in
determining the quantity of certain direct factors of the habitat. Perhaps a
more important connection between physiography and ecology is to be
found in succession. The beginning of all primary, and of many secondary
successions is to be sought in the physiographic processes which produce
new habitats, or modify old ones. On the other hand, most of the reactions
which continue successions exert a direct influence upon the form of the
land. The most pronounced influence of terrestrial successions is found in
the stabilization which their ultimate stages exert upon land forms, even
where these are highly immature. The chief effect of aquatic successions is
to be found in the “silting up” and the formation of new land, which result
from the action of vegetation upon silt-bearing waters. The closeness of the
relation between succession and the forms of the land has led to the
application of the term “physiographic ecology” to that part of the subject
which deals with the development of vegetation, i. e., succession.
22. Soil physics. This subject is as much a part of ecology as is forestry.
It is intrinsically that subdivision of ecology which deals with the edaphic
factors of the habitat, and their relation to the plant. Since the basis is
physics, there has been a general tendency to overvalue the determination
of soil properties, and to ignore the fact that these are decisive only when
considered with reference to the living plant. As the soil contains the water
which is the factor of greatest importance to plants, soil physics is an
especially important part of ecology. Its methods are discussed under the
habitat.
23. Zoogeography. Since animals are free for the most part, and hence
not confined so strictly to one spot as plants, their dependence upon the
habitat is not so evident. The relation is further obscured by the fact that no
physical factor has the direct effect upon them which water or light exerts
upon the plant. Vegetation, indeed, as the source of food and protection,
to ascertain what non-forested areas offer the most nearly similar
conditions. A knowledge of habitats and the means of measuring them
enables the forester to discover the causes which control the vegetation with
which he is already familiar, and to forecast results otherwise hidden.
Furthermore, it makes it possible for him to enter a new region and to
determine its nature and capabilities at a minimum of time and energy.
21. Physiography. Physiographic features play an important part in
determining the quantity of certain direct factors of the habitat. Perhaps a
more important connection between physiography and ecology is to be
found in succession. The beginning of all primary, and of many secondary
successions is to be sought in the physiographic processes which produce
new habitats, or modify old ones. On the other hand, most of the reactions
which continue successions exert a direct influence upon the form of the
land. The most pronounced influence of terrestrial successions is found in
the stabilization which their ultimate stages exert upon land forms, even
where these are highly immature. The chief effect of aquatic successions is
to be found in the “silting up” and the formation of new land, which result
from the action of vegetation upon silt-bearing waters. The closeness of the
relation between succession and the forms of the land has led to the
application of the term “physiographic ecology” to that part of the subject
which deals with the development of vegetation, i. e., succession.
22. Soil physics. This subject is as much a part of ecology as is forestry.
It is intrinsically that subdivision of ecology which deals with the edaphic
factors of the habitat, and their relation to the plant. Since the basis is
physics, there has been a general tendency to overvalue the determination
of soil properties, and to ignore the fact that these are decisive only when
considered with reference to the living plant. As the soil contains the water
which is the factor of greatest importance to plants, soil physics is an
especially important part of ecology. Its methods are discussed under the
habitat.
23. Zoogeography. Since animals are free for the most part, and hence
not confined so strictly to one spot as plants, their dependence upon the
habitat is not so evident. The relation is further obscured by the fact that no
physical factor has the direct effect upon them which water or light exerts
upon the plant. Vegetation, indeed, as the source of food and protection,
Page 42
plays a more obvious, if not a more important part. This is especially true of
anthophilous insects, but it also holds for all herbivorous animals, and,
through them, for carnivorous ones. The animal ecology of a particular
region can only be properly investigated after the habitats and plant
formations have been carefully studied. Here, as in floristics, a great deal
can be done in the way of listing the fauna, or studying the life habits of its
species, without any knowledge of plant ecology; but an adequate study
must be based upon a knowledge of the vegetation. Although animal
formations are often poorly defined, there can be no doubt of their
existence. Frequently they coincide with plant formations, and then have
very definite limits. They exhibit both development and structure, and are
subject to the laws of invasion, succession, zonation, and alternation,
though these are not altogether similar to those known for plants, a fact
readily explained by the motility of animals. Considered from the above
point of view, zoogeography is a virgin field, and it promises great things to
the student who approaches it with the proper training.
24. Sociology. In its fundamental aspects, sociology is the ecology of a
particular species of animal, and has in consequence, a similar close
connection with plant ecology. The widespread migration of man and his
social nature have resulted in the production of groups or communities
which have much more in common with plant formations than do
formations of other animals. The laws of association apply with especial
force to the family, tribe, community, etc., while the laws of succession are
essentially the same for both plants and man. At first thought it might seem
that man’s ability to change his dwelling-place and to modify his
environment exempts him in large measure from the influence of the
habitat. The exemption, however, is only apparent, as the control exerted by
climate, soil, and physiography is all but absolute, particularly when man’s
dependence upon vegetation, both natural and cultural, is called to mind.
The Essentials of a System
25. Cause and effect: habitat and plant. In seeking to lay the
foundation for a broad and thorough system of ecological research, it is
necessary to scan the whole field, and to discriminate carefully between
what is fundamental and what is merely collateral. The chief task is to
anthophilous insects, but it also holds for all herbivorous animals, and,
through them, for carnivorous ones. The animal ecology of a particular
region can only be properly investigated after the habitats and plant
formations have been carefully studied. Here, as in floristics, a great deal
can be done in the way of listing the fauna, or studying the life habits of its
species, without any knowledge of plant ecology; but an adequate study
must be based upon a knowledge of the vegetation. Although animal
formations are often poorly defined, there can be no doubt of their
existence. Frequently they coincide with plant formations, and then have
very definite limits. They exhibit both development and structure, and are
subject to the laws of invasion, succession, zonation, and alternation,
though these are not altogether similar to those known for plants, a fact
readily explained by the motility of animals. Considered from the above
point of view, zoogeography is a virgin field, and it promises great things to
the student who approaches it with the proper training.
24. Sociology. In its fundamental aspects, sociology is the ecology of a
particular species of animal, and has in consequence, a similar close
connection with plant ecology. The widespread migration of man and his
social nature have resulted in the production of groups or communities
which have much more in common with plant formations than do
formations of other animals. The laws of association apply with especial
force to the family, tribe, community, etc., while the laws of succession are
essentially the same for both plants and man. At first thought it might seem
that man’s ability to change his dwelling-place and to modify his
environment exempts him in large measure from the influence of the
habitat. The exemption, however, is only apparent, as the control exerted by
climate, soil, and physiography is all but absolute, particularly when man’s
dependence upon vegetation, both natural and cultural, is called to mind.
The Essentials of a System
25. Cause and effect: habitat and plant. In seeking to lay the
foundation for a broad and thorough system of ecological research, it is
necessary to scan the whole field, and to discriminate carefully between
what is fundamental and what is merely collateral. The chief task is to
Page 43
discover, if possible, such a guiding principle as will furnish a basis for a
permanent and logical superstructure. In ecology, the one relation which is
precedent to all others is the one that exists between the habitat and the
plant. This relation has long been known, but its full value has yet to be
appreciated. It is precisely the relation that exists between cause and effect,
and its fundamental importance lies in the fact that all questions concerning
the plant lead back to it ultimately. Other relations are important, but no
other is paramount, or able to serve as the basis of ecology. Ecology sums
up this relation of cause and effect in a single word, and it may be that this
advantage will finally cause its general acceptance as the proper name for
this great field.
In the further analysis of the connection between the habitat and the
plant, it is evident that the causes or factors of the habitat act directly upon
the plant as an individual, and at the same time upon plants as groups of
individuals. The latter in no wise decreases the importance of the plant as
the primary effect of the habitat, but it gives form to research by making it
possible to consider two great natural groups of phenomena, each
characterized by very different categories of effects. Ecology thus falls
naturally into three great fundamental fields of inquiry: habitat, plant, and
formation (or vegetation). To be sure, the last can be approached only
through the plant, but as the latter is not an individual, but the unit of a
complex from the formational standpoint, the formation itself may be
regarded as a sort of multiple organism, which is in many ways at least a
direct effect of the habitat. In emphasizing this fundamental relation of
habitat and vegetation, it is imperative not to ignore the fact that neither
plant nor formation is altogether the effect of its present habitat. A third
element must always be considered, namely, the historical fact, by which is
meant the ancestral structure. Upon analysis, however, this is in its turn
found to be the product of antecedent habitats, and in consequence the
essential connection between the habitat and the plant is seen to be absolute.
26. The place of function. In the foregoing it is understood that the
immediate effect of the physical factors of the habitat is to be found in the
functions of the plant, and that these determine the plant structure. Function
has so long been the especial theme of plant physiology that methods of
investigation are numerous and well known, and it is unnecessary here to
consider it further than to indicate its general bearing. The essential
sequence in ecological research, then, is the one already indicated, viz.,
permanent and logical superstructure. In ecology, the one relation which is
precedent to all others is the one that exists between the habitat and the
plant. This relation has long been known, but its full value has yet to be
appreciated. It is precisely the relation that exists between cause and effect,
and its fundamental importance lies in the fact that all questions concerning
the plant lead back to it ultimately. Other relations are important, but no
other is paramount, or able to serve as the basis of ecology. Ecology sums
up this relation of cause and effect in a single word, and it may be that this
advantage will finally cause its general acceptance as the proper name for
this great field.
In the further analysis of the connection between the habitat and the
plant, it is evident that the causes or factors of the habitat act directly upon
the plant as an individual, and at the same time upon plants as groups of
individuals. The latter in no wise decreases the importance of the plant as
the primary effect of the habitat, but it gives form to research by making it
possible to consider two great natural groups of phenomena, each
characterized by very different categories of effects. Ecology thus falls
naturally into three great fundamental fields of inquiry: habitat, plant, and
formation (or vegetation). To be sure, the last can be approached only
through the plant, but as the latter is not an individual, but the unit of a
complex from the formational standpoint, the formation itself may be
regarded as a sort of multiple organism, which is in many ways at least a
direct effect of the habitat. In emphasizing this fundamental relation of
habitat and vegetation, it is imperative not to ignore the fact that neither
plant nor formation is altogether the effect of its present habitat. A third
element must always be considered, namely, the historical fact, by which is
meant the ancestral structure. Upon analysis, however, this is in its turn
found to be the product of antecedent habitats, and in consequence the
essential connection between the habitat and the plant is seen to be absolute.
26. The place of function. In the foregoing it is understood that the
immediate effect of the physical factors of the habitat is to be found in the
functions of the plant, and that these determine the plant structure. Function
has so long been the especial theme of plant physiology that methods of
investigation are numerous and well known, and it is unnecessary here to
consider it further than to indicate its general bearing. The essential
sequence in ecological research, then, is the one already indicated, viz.,
Page 44
habitat, plant, and formation, and this will constitute the order of treatment
in the following pages. That portion of floristic which is not mere
descriptive botany belongs to the consideration of the formation, and in
consequence there will be no special treatment of floristic as a subdivision
of ecology.
in the following pages. That portion of floristic which is not mere
descriptive botany belongs to the consideration of the formation, and in
consequence there will be no special treatment of floristic as a subdivision
of ecology.
Page 45
CHAPTER II. THE HABITAT
Concept and Analysis
27. Definition of the habitat. The habitat is the sum of all the forces or factors present
in a given area. It is the exact equivalent of the term environment, though the latter is
commonly used in a more general sense. As an ecological concept, the habitat refers to
an area much more definite in character, and more sharply limited in extent than the
habitat of species as indicated in the manuals. Since the careful study of habitats has
scarcely begun, it is impossible to recognize and delimit them in an absolute sense.
Visible topographic boundaries often exist, but in many cases, the limit, though actual, is
not readily perceived. Contiguous habitats may be sharply limited, or they may pass into
each other so gradually that no real line of demarcation can be drawn. Whatever
variations they may show, however, all habitats agree in the possession of certain
essential factors, which are universally present. On the other hand, a few factors are
merely incidental and may be present or absent. The relative value and amount of these is
probably similar for no two habitats, though the latter readily fall into groups with
reference to the amount of some particular factor.
28. Factors. The factors of a habitat are water-content, humidity, light, temperature,
soil, wind, precipitation, pressure, altitude, exposure, slope, surface (cover), and animals.
To these should be added gravity and polarity, which are practically uniform for all
habitats, and may, in consequence, be ignored in this treatise. Length of season, while it
plays an important part in vegetation, is clearly a complex and is to be treated under its
constituents. Of the factors given, all are regularly found in each habitat, though some
are not constantly present. The first five, water-content, humidity, light, temperature, and
soil are the most important, and any one may well serve as a basis for grouping habitats
into particular classes with reference to quantity. As will be pointed out later, however,
water-content and light furnish the most striking differences between habitats, and offer
the best means of classification. As habitats are inseparable from the formations which
they bear, the discussion of the kinds of habitats is reserved for chapter IV.
Concept and Analysis
27. Definition of the habitat. The habitat is the sum of all the forces or factors present
in a given area. It is the exact equivalent of the term environment, though the latter is
commonly used in a more general sense. As an ecological concept, the habitat refers to
an area much more definite in character, and more sharply limited in extent than the
habitat of species as indicated in the manuals. Since the careful study of habitats has
scarcely begun, it is impossible to recognize and delimit them in an absolute sense.
Visible topographic boundaries often exist, but in many cases, the limit, though actual, is
not readily perceived. Contiguous habitats may be sharply limited, or they may pass into
each other so gradually that no real line of demarcation can be drawn. Whatever
variations they may show, however, all habitats agree in the possession of certain
essential factors, which are universally present. On the other hand, a few factors are
merely incidental and may be present or absent. The relative value and amount of these is
probably similar for no two habitats, though the latter readily fall into groups with
reference to the amount of some particular factor.
28. Factors. The factors of a habitat are water-content, humidity, light, temperature,
soil, wind, precipitation, pressure, altitude, exposure, slope, surface (cover), and animals.
To these should be added gravity and polarity, which are practically uniform for all
habitats, and may, in consequence, be ignored in this treatise. Length of season, while it
plays an important part in vegetation, is clearly a complex and is to be treated under its
constituents. Of the factors given, all are regularly found in each habitat, though some
are not constantly present. The first five, water-content, humidity, light, temperature, and
soil are the most important, and any one may well serve as a basis for grouping habitats
into particular classes with reference to quantity. As will be pointed out later, however,
water-content and light furnish the most striking differences between habitats, and offer
the best means of classification. As habitats are inseparable from the formations which
they bear, the discussion of the kinds of habitats is reserved for chapter IV.
Page 46
Classification of Factors
29. The nature of factors. The factors of a habitat are arranged in two groups
according to their nature: (1) physical, (2) biotic. In the strict sense, the physical factors
constitute the habitat proper, and are the real causative forces. No habitat escapes the
influence of biotic factors, however, as the formation always reacts upon it, and the
influence of animals is usually felt in some measure. Physical factors are further grouped
into (1) climatic and (2) edaphic, with respect to source, or, better, the medium in which
they are found. Climatic, or atmospheric factors are humidity, light, temperature, wind,
pressure, and precipitation. Axiomatically, the stimuli which they produce are especially
related to the leaf. Edaphic or soil factors are confined to the soil, as the term denotes,
and are immediately concerned with the functions of the root. Water-content is by far the
most important of these; the others are soil composition (nutrient-content), soil
temperature, altitude, slope, exposure, and surface. The last four are of a more general
character than the others, and are usually referred to as physiographic factors. Cover,
when dead, might well be placed among these also, but as it is little different from the
living cover in effect, it seems most logical to refer it to biotic factors.
30. The influence of factors. While the above classification is both obvious and
convenient, a more logical and intimate grouping may be made upon the influence which
the factor exerts. On this basis, factors are divided into (1) direct, (2) indirect, and (3)
remote. Direct factors are those which act directly upon an important function of the
plant and produce a formative effect: for example, an increase in humidity produces an
immediate decrease in transpiration. They are water-content, humidity, and light. Other
factors have a direct action: thus temperature has an immediate influence upon
respiration and probably assimilation also, but it is not structurally formative. Wind has a
direct mechanical effect upon woody plants, but it does not fall within our definition.
Indirect factors are those that affect a formative function of the plant through another
factor; thus a change in temperature causes a change in humidity and this in turn calls
forth a change in transpiration; or, a change in soil texture increases the water-content,
and this affects the imbibition of the root-hairs. Indirect factors, then, are temperature,
wind, pressure, precipitation, and soil composition. Remote factors are, for the most part,
physiographic and biotic: they require at least two other factors to act as middlemen.
Altitude affects plants through pressure, which modifies humidity, and hence
transpiration. Slope determines in large degree the run-off during a rainstorm, thus
affecting water-content and the amount of water absorbed. Earthworms and plant parts
change the texture of the soil, and thereby the water-content. Indirect factors often exert a
remote influence also, as may be seen in the effect which temperature and wind have in
increasing evaporation from the soil, and thus reducing the water-content. This
distinction between factors may seem insufficiently grounded. In this event, it should be
noted that it centers the effects of all factors upon the three direct ones, water-content,
humidity, and light. If it further be recalled that these are the only factors which produce
29. The nature of factors. The factors of a habitat are arranged in two groups
according to their nature: (1) physical, (2) biotic. In the strict sense, the physical factors
constitute the habitat proper, and are the real causative forces. No habitat escapes the
influence of biotic factors, however, as the formation always reacts upon it, and the
influence of animals is usually felt in some measure. Physical factors are further grouped
into (1) climatic and (2) edaphic, with respect to source, or, better, the medium in which
they are found. Climatic, or atmospheric factors are humidity, light, temperature, wind,
pressure, and precipitation. Axiomatically, the stimuli which they produce are especially
related to the leaf. Edaphic or soil factors are confined to the soil, as the term denotes,
and are immediately concerned with the functions of the root. Water-content is by far the
most important of these; the others are soil composition (nutrient-content), soil
temperature, altitude, slope, exposure, and surface. The last four are of a more general
character than the others, and are usually referred to as physiographic factors. Cover,
when dead, might well be placed among these also, but as it is little different from the
living cover in effect, it seems most logical to refer it to biotic factors.
30. The influence of factors. While the above classification is both obvious and
convenient, a more logical and intimate grouping may be made upon the influence which
the factor exerts. On this basis, factors are divided into (1) direct, (2) indirect, and (3)
remote. Direct factors are those which act directly upon an important function of the
plant and produce a formative effect: for example, an increase in humidity produces an
immediate decrease in transpiration. They are water-content, humidity, and light. Other
factors have a direct action: thus temperature has an immediate influence upon
respiration and probably assimilation also, but it is not structurally formative. Wind has a
direct mechanical effect upon woody plants, but it does not fall within our definition.
Indirect factors are those that affect a formative function of the plant through another
factor; thus a change in temperature causes a change in humidity and this in turn calls
forth a change in transpiration; or, a change in soil texture increases the water-content,
and this affects the imbibition of the root-hairs. Indirect factors, then, are temperature,
wind, pressure, precipitation, and soil composition. Remote factors are, for the most part,
physiographic and biotic: they require at least two other factors to act as middlemen.
Altitude affects plants through pressure, which modifies humidity, and hence
transpiration. Slope determines in large degree the run-off during a rainstorm, thus
affecting water-content and the amount of water absorbed. Earthworms and plant parts
change the texture of the soil, and thereby the water-content. Indirect factors often exert a
remote influence also, as may be seen in the effect which temperature and wind have in
increasing evaporation from the soil, and thus reducing the water-content. This
distinction between factors may seem insufficiently grounded. In this event, it should be
noted that it centers the effects of all factors upon the three direct ones, water-content,
humidity, and light. If it further be recalled that these are the only factors which produce
Page 47
qualitative structural changes, and that the classification of ecads and formations is based
upon them, the validity of the distinction is clear.
upon them, the validity of the distinction is clear.
Page 48
The Determination of Factors
31. The need of exact measurement. Any serious endeavor to find in the habitat
those causes which are producing modification in the plant and in vegetation can not stop
with the factors merely. The next step is to determine the quantity of each. It is not
sufficient to hazard a guess at this, or to make a rough estimate of it. Habitats differ in all
degrees, and it is impossible to institute comparisons between them without an exact
measure of each factor. Similarly, one can not trace the adaptations of species to their
proper causes unless the quantity of each factor is known. It is of little value to know the
general effect of a factor, unless it is known to what degree this effect is exerted. For this
purpose it becomes necessary to appeal to instruments, in order to determine the exact
amount of each factor that is present in a particular habitat, and hence to determine the
ratio between the stimulus and the amount of structural adjustment which results. The
employment of instruments of precision is clearly indispensable for the task which we
have set for ecology, and every student that intends to strike at the root of the subject,
and to make lasting contributions to it, must familiarize himself with instrumental
methods. One great benefit will accrue to ecology as soon as this fact is generally
recognized. The use of instruments and the application of results obtained from them
demand much patience and seriousness of purpose upon the part of the student. As a
consequence, there will be a general exodus from ecology of those that have been
attracted to it as the latest botanical fad, and have done so much to bring it into disrepute.
32. The value of meteorological methods. At the outset there must be a very clear
understanding that weather records and readings have only a very general value. This is
in spite of the fact that the instruments employed are of standard precision. An important
reason for this lack of value is that readings are not made in a particular habitat; as a rule,
indeed, they are made in towns and cities, and hence are far removed from masses of
vegetation. They are usually taken at considerable heights, and give but a general
indication of the conditions at the level of vegetation. The chief difficulty, however, is
that the factors observed at weather stations—temperature, pressure, wind, and
precipitation—are those which have the least value for the ecologist. It is true that a
knowledge of the temperature and rainfall of a great region will afford some idea of the
general character of its vegetation. A proper understanding of such a vegetation is,
however, to be gained only through the exact study of its component formations. Ecology
has already incurred sufficient censure as a subject composed of very general ideas, and
the use of meteorological data, which can never be connected definitely with anything in
the plant or the formation, should be discontinued. This must not be understood to mean
that meteorological instruments can not be used in the proper place and manner, i. e., in
the habitat.
33. Habitat determination. It is self-evident that determinations of factors by
instruments can only be of value in the habitat where they are made. In other words, a
habitat is a unit for purposes of measuring its factors, and measures of one habitat have
31. The need of exact measurement. Any serious endeavor to find in the habitat
those causes which are producing modification in the plant and in vegetation can not stop
with the factors merely. The next step is to determine the quantity of each. It is not
sufficient to hazard a guess at this, or to make a rough estimate of it. Habitats differ in all
degrees, and it is impossible to institute comparisons between them without an exact
measure of each factor. Similarly, one can not trace the adaptations of species to their
proper causes unless the quantity of each factor is known. It is of little value to know the
general effect of a factor, unless it is known to what degree this effect is exerted. For this
purpose it becomes necessary to appeal to instruments, in order to determine the exact
amount of each factor that is present in a particular habitat, and hence to determine the
ratio between the stimulus and the amount of structural adjustment which results. The
employment of instruments of precision is clearly indispensable for the task which we
have set for ecology, and every student that intends to strike at the root of the subject,
and to make lasting contributions to it, must familiarize himself with instrumental
methods. One great benefit will accrue to ecology as soon as this fact is generally
recognized. The use of instruments and the application of results obtained from them
demand much patience and seriousness of purpose upon the part of the student. As a
consequence, there will be a general exodus from ecology of those that have been
attracted to it as the latest botanical fad, and have done so much to bring it into disrepute.
32. The value of meteorological methods. At the outset there must be a very clear
understanding that weather records and readings have only a very general value. This is
in spite of the fact that the instruments employed are of standard precision. An important
reason for this lack of value is that readings are not made in a particular habitat; as a rule,
indeed, they are made in towns and cities, and hence are far removed from masses of
vegetation. They are usually taken at considerable heights, and give but a general
indication of the conditions at the level of vegetation. The chief difficulty, however, is
that the factors observed at weather stations—temperature, pressure, wind, and
precipitation—are those which have the least value for the ecologist. It is true that a
knowledge of the temperature and rainfall of a great region will afford some idea of the
general character of its vegetation. A proper understanding of such a vegetation is,
however, to be gained only through the exact study of its component formations. Ecology
has already incurred sufficient censure as a subject composed of very general ideas, and
the use of meteorological data, which can never be connected definitely with anything in
the plant or the formation, should be discontinued. This must not be understood to mean
that meteorological instruments can not be used in the proper place and manner, i. e., in
the habitat.
33. Habitat determination. It is self-evident that determinations of factors by
instruments can only be of value in the habitat where they are made. In other words, a
habitat is a unit for purposes of measuring its factors, and measures of one habitat have
Page 49
no exact value in another. This fact can not be overstated. Thus, while it is perfectly
legitimate, and indeed highly desirable, to locate thermographs in different mountain
zones for ascertaining the rate at which temperature decreases with altitude, the data
obtained in this way are not directly applicable in explanation of plant or formation
changes, except when the same species occurs at different altitudes. Special methods are
valuable and often absolutely necessary, but in view of the fact that the plant as well as
the formation is the definite product of a definite habitat, the fundamental rule in
instrumentation is that complete readings must be made within a habitat for that habitat
alone. This necessarily presupposes a certain preliminary acquaintance with the habitat to
be investigated, as it is imperative that the station for making readings be located well
within the formation, in order to avoid transition conditions. In vegetation, there are as
many habitats as formations, and in addition a large number of new and denuded habitats
upon which successions have not yet started; a knowledge of each formation or
succession must rest ultimately upon the factors of its particular habitat.
34. Determinable and efficient differences. The instruments employed in studying
habitats can not be too exact, as there is no adequate knowledge as yet concerning the
real differences which exist between related or contiguous formations. This is
particularly true of differences which are efficient in producing a recognizable structural
change in plant or formation. Investigations made by the writer have shewn that standard
instruments will measure differences of quantity quite too small to produce a visible
reaction. Efficient differences are not the same for different factors, and perhaps also for
the same factor when found in various combinations. They vary widely for different
species, being in direct relation to the plasticity of the latter. The point necessary to bear
in mind in formulating methods for habitat investigation and in making use of
instruments is that standard instruments should be used for the very reason that we do not
yet know the relation between determinable and efficient differences. On the other hand,
it is unnecessary to insist upon absolute exactness as soon as it is found that the
determinable difference lies well within the efficient one. This by no means indicates that
instruments are not to be carefully standardized and frequently checked, or that accurate
readings should not be made. It means that a slight margin of error may be permitted in a
machine which registers well within the efficient difference for that factor, and that
instruments that read to the last degree of nicety are not absolutely necessary. In the
fundamental work of determining efficient differences, however, instruments can not
have too great precision. Moreover, these differences must be based upon the most
plastic species of a formation, and the readings must be made under normal conditions.
legitimate, and indeed highly desirable, to locate thermographs in different mountain
zones for ascertaining the rate at which temperature decreases with altitude, the data
obtained in this way are not directly applicable in explanation of plant or formation
changes, except when the same species occurs at different altitudes. Special methods are
valuable and often absolutely necessary, but in view of the fact that the plant as well as
the formation is the definite product of a definite habitat, the fundamental rule in
instrumentation is that complete readings must be made within a habitat for that habitat
alone. This necessarily presupposes a certain preliminary acquaintance with the habitat to
be investigated, as it is imperative that the station for making readings be located well
within the formation, in order to avoid transition conditions. In vegetation, there are as
many habitats as formations, and in addition a large number of new and denuded habitats
upon which successions have not yet started; a knowledge of each formation or
succession must rest ultimately upon the factors of its particular habitat.
34. Determinable and efficient differences. The instruments employed in studying
habitats can not be too exact, as there is no adequate knowledge as yet concerning the
real differences which exist between related or contiguous formations. This is
particularly true of differences which are efficient in producing a recognizable structural
change in plant or formation. Investigations made by the writer have shewn that standard
instruments will measure differences of quantity quite too small to produce a visible
reaction. Efficient differences are not the same for different factors, and perhaps also for
the same factor when found in various combinations. They vary widely for different
species, being in direct relation to the plasticity of the latter. The point necessary to bear
in mind in formulating methods for habitat investigation and in making use of
instruments is that standard instruments should be used for the very reason that we do not
yet know the relation between determinable and efficient differences. On the other hand,
it is unnecessary to insist upon absolute exactness as soon as it is found that the
determinable difference lies well within the efficient one. This by no means indicates that
instruments are not to be carefully standardized and frequently checked, or that accurate
readings should not be made. It means that a slight margin of error may be permitted in a
machine which registers well within the efficient difference for that factor, and that
instruments that read to the last degree of nicety are not absolutely necessary. In the
fundamental work of determining efficient differences, however, instruments can not
have too great precision. Moreover, these differences must be based upon the most
plastic species of a formation, and the readings must be made under normal conditions.
Page 50
Instrumentation
35. Methods. In the field use of instruments two methods have been developed. The
first in point of time was the method of simple instruments, devised especially for class
work, and capable of being used only where a number of trained students are available.
The method of automatic instruments was an immediate outgrowth of this, due to the
necessity which confronts the solitary investigator of being in different habitats at the
same time. In the gradual evolution of this subject, it has become possible to combine the
two methods in such a way as to retain all the advantages of the automatic method, and
most of those of the method of simple instruments.
36. Method of simple instruments. By simple instruments are denoted those that do
not record, but must be read by the observer at the time. They are standard instruments of
precision, but possess the disadvantage of requiring an observer for each one. They are
well illustrated by the thermometers and psychrometers used by the Weather Bureau. In
the hands of trained observers the results obtained are unimpeachable; in fact, standard
simple instruments must be constantly employed to check automatic ones. As physical
factors vary greatly through the day and through the year, it is all-important that the
readings in habitats which are being compared should be made at the same instant. This
requires a number of observers; as many as twelve stations have been read at one time,
and there is of course no limit to the number. It is very important, also, that observers be
carefully trained in the handling of instruments, and in reading them accurately and
intelligently at the proper moment. In practice it has been found impossible to do such
work in elementary classes, and, even in using small advanced classes, prolonged drill
has been necessary before trustworthy results could be obtained. When a class has once
been thoroughly trained in making accurate simultaneous readings, there is practically no
limit, other than that set by time, to the valuable work that can be done, both in
instruction and investigation.
37. Method of automatic instruments. The solitary investigator must replace trained
helpers by automatic instruments or ecographs. These have the very great advantages of
giving continuous simultaneous records for long periods, and of having no personal
equation. They must be regulated and checked, to be sure, but as this is all done by the
same person, the error is negligible. There is nothing more satisfactory in resident
investigation than a series of accurate recording instruments in various habitats.
Ecographs have two disadvantages. The chief perhaps is cost. The expense of a single
“battery” which will record light, water-content, humidity, and temperature is about
$250. Another difficulty is that they can be used only within a few miles of the base,
since they require attention every week for regulation, change of record, etc. While this
means that ecographs in their present form are not adapted to reconnaissance, this is not a
real disadvantage, as the scattered observations possible on such a journey can best be
made by simple instruments.
35. Methods. In the field use of instruments two methods have been developed. The
first in point of time was the method of simple instruments, devised especially for class
work, and capable of being used only where a number of trained students are available.
The method of automatic instruments was an immediate outgrowth of this, due to the
necessity which confronts the solitary investigator of being in different habitats at the
same time. In the gradual evolution of this subject, it has become possible to combine the
two methods in such a way as to retain all the advantages of the automatic method, and
most of those of the method of simple instruments.
36. Method of simple instruments. By simple instruments are denoted those that do
not record, but must be read by the observer at the time. They are standard instruments of
precision, but possess the disadvantage of requiring an observer for each one. They are
well illustrated by the thermometers and psychrometers used by the Weather Bureau. In
the hands of trained observers the results obtained are unimpeachable; in fact, standard
simple instruments must be constantly employed to check automatic ones. As physical
factors vary greatly through the day and through the year, it is all-important that the
readings in habitats which are being compared should be made at the same instant. This
requires a number of observers; as many as twelve stations have been read at one time,
and there is of course no limit to the number. It is very important, also, that observers be
carefully trained in the handling of instruments, and in reading them accurately and
intelligently at the proper moment. In practice it has been found impossible to do such
work in elementary classes, and, even in using small advanced classes, prolonged drill
has been necessary before trustworthy results could be obtained. When a class has once
been thoroughly trained in making accurate simultaneous readings, there is practically no
limit, other than that set by time, to the valuable work that can be done, both in
instruction and investigation.
37. Method of automatic instruments. The solitary investigator must replace trained
helpers by automatic instruments or ecographs. These have the very great advantages of
giving continuous simultaneous records for long periods, and of having no personal
equation. They must be regulated and checked, to be sure, but as this is all done by the
same person, the error is negligible. There is nothing more satisfactory in resident
investigation than a series of accurate recording instruments in various habitats.
Ecographs have two disadvantages. The chief perhaps is cost. The expense of a single
“battery” which will record light, water-content, humidity, and temperature is about
$250. Another difficulty is that they can be used only within a few miles of the base,
since they require attention every week for regulation, change of record, etc. While this
means that ecographs in their present form are not adapted to reconnaissance, this is not a
real disadvantage, as the scattered observations possible on such a journey can best be
made by simple instruments.
Page 51
38. Combined methods. The best results by far are to be obtained by the combined
use of simple and automatic instruments. This is particularly true in research, but it
applies also to class instruction. The ecographs afford a continuous, accurate basal
record, to which a single reading made at any time or place can be readily referred for
comparison. On the other hand, it is an easy matter to carry a full complement of simple
instruments on the daily field trips, and to make accurate readings in a score or more of
formations in a single day. An isolated reading, especially of a climatic factor, has little
or no value in itself, but when it can be compared with a reading made at the same time
in the base station by an ecograph, it is the equivalent of an automatic reading. This
method renders a set of simple instruments more desirable for a long trip or
reconnaissance than a battery of automatic ones. It is practically impossible to carry the
latter into the field, and in any event a continuous record is out of the question. As there
are other tasks at such times also, it becomes evident that the taking of single readings
which can be compared with a continuous record offers the most satisfactory solution.
use of simple and automatic instruments. This is particularly true in research, but it
applies also to class instruction. The ecographs afford a continuous, accurate basal
record, to which a single reading made at any time or place can be readily referred for
comparison. On the other hand, it is an easy matter to carry a full complement of simple
instruments on the daily field trips, and to make accurate readings in a score or more of
formations in a single day. An isolated reading, especially of a climatic factor, has little
or no value in itself, but when it can be compared with a reading made at the same time
in the base station by an ecograph, it is the equivalent of an automatic reading. This
method renders a set of simple instruments more desirable for a long trip or
reconnaissance than a battery of automatic ones. It is practically impossible to carry the
latter into the field, and in any event a continuous record is out of the question. As there
are other tasks at such times also, it becomes evident that the taking of single readings
which can be compared with a continuous record offers the most satisfactory solution.
Page 52
Construction and Use of Instruments
39. The selection of instruments. In selecting and devising instruments for the
investigation of physical factors, emphasis has first been laid upon accuracy. This is the
result of a feeling that it is better to have instruments that read too minutely than those
which do not make distinctions that are sufficiently close, particularly until more has
been learned about efficient differences. On the other hand, no hesitation has been felt in
employing instruments which are not absolutely accurate, when it was clear that the error
was less than the efficient difference. Similarly, the margin of error practically eliminates
itself in the case of simultaneous comparative readings, when the instruments have been
checked to the same standard. Simplicity of construction and operation are of great
importance, especially in saving time where a large number of instruments are in
operation. Expense is likewise to be carefully considered. It is impossible to have too
many instruments, but cost practically determines the number that can be obtained. It is
further necessary to secure or invent both simple and automatic instruments for all
factors, except such invariable ones as altitude, slope, etc. Simple instruments must be of
a kind that can be easily carried, and so constructed that they can be used at a minimum
of risk. The sling psychrometer, for example, is very readily broken in field use, and it
has been replaced by a protected modification, the rotating form.
In describing the construction and operation of the many factor instruments, there has
been no attempt to make the treatment exhaustive. Those instruments which the author
has found of greatest value in his own work are given precedence, and the manner of
using them is described in detail. Other instruments of value are also considered, though
with greater brevity. Some of the most complex and expensive ones have been ignored,
as it is altogether improbable that they can come into general use in their present form.
While the conviction is felt that the methods described below will enable the most
advanced investigators to carry on thorough work, it is hoped that they will be seen to be
so fundamental, and so attractive, that they will appeal to all who are planning serious
ecological study.
39. The selection of instruments. In selecting and devising instruments for the
investigation of physical factors, emphasis has first been laid upon accuracy. This is the
result of a feeling that it is better to have instruments that read too minutely than those
which do not make distinctions that are sufficiently close, particularly until more has
been learned about efficient differences. On the other hand, no hesitation has been felt in
employing instruments which are not absolutely accurate, when it was clear that the error
was less than the efficient difference. Similarly, the margin of error practically eliminates
itself in the case of simultaneous comparative readings, when the instruments have been
checked to the same standard. Simplicity of construction and operation are of great
importance, especially in saving time where a large number of instruments are in
operation. Expense is likewise to be carefully considered. It is impossible to have too
many instruments, but cost practically determines the number that can be obtained. It is
further necessary to secure or invent both simple and automatic instruments for all
factors, except such invariable ones as altitude, slope, etc. Simple instruments must be of
a kind that can be easily carried, and so constructed that they can be used at a minimum
of risk. The sling psychrometer, for example, is very readily broken in field use, and it
has been replaced by a protected modification, the rotating form.
In describing the construction and operation of the many factor instruments, there has
been no attempt to make the treatment exhaustive. Those instruments which the author
has found of greatest value in his own work are given precedence, and the manner of
using them is described in detail. Other instruments of value are also considered, though
with greater brevity. Some of the most complex and expensive ones have been ignored,
as it is altogether improbable that they can come into general use in their present form.
While the conviction is felt that the methods described below will enable the most
advanced investigators to carry on thorough work, it is hoped that they will be seen to be
so fundamental, and so attractive, that they will appeal to all who are planning serious
ecological study.
Page 53
WATER-CONTENT
40. Value of different instruments. The paramount importance of water-content as a
direct factor in the modification of plant form and distribution gives a fundamental value
to the methods used for its determination. Automatic instruments for ascertaining the
water in the soil are costly, in addition to being complicated, and often inaccurate. For
these reasons, much attention has been given to developing the simpler but more reliable
methods in which a soil borer or geotome is used. The latter is simple, inexpensive, and
accurate. It can be carried easily upon daily trips or upon longer reconnaissances, and is
always ready for instant use. In the determination of physiological water-content, it is
practically indispensable. Indeed, the readiness with which geotome determinations of
water-content can be made should hasten the universal recognition of the fact that it is
the available, and not the total amount of water in the soil, which determines the effect
upon the plant.
Geotome Methods
41. The geotome. In its simplest form,
the geotome is merely a stout iron tube
with a sharp cutting edge at one end and a
firmly attached handle at the other. The
length is variable and is primarily
determined by the location of the active
root surface of the plant. In xerophytic
habitats, generally a longer tube is
necessary than in mesophytic ones. The
bore is largely determined by the
character of the soil; for example, a larger
one is necessary for gravel than for loam.
Tubes of small bore also tend to pack the
soil below them, and to give a
correspondingly incomplete core. The
best results have been obtained with
geotomes of ½–1 inch tube. Each
geotome has a removable rod, flattened
into a disk at one end, and bent at the
other, for forcing out the core after it has
been cut from the soil. Sets of geotomes Fig. 1. Geotomes and soil can.
have been made in lengths of 5, 10, 12,
15, 20, and 25 inches. The 12– and 15–
inch forms have been commonly used for herbaceous formations and layers. They are
40. Value of different instruments. The paramount importance of water-content as a
direct factor in the modification of plant form and distribution gives a fundamental value
to the methods used for its determination. Automatic instruments for ascertaining the
water in the soil are costly, in addition to being complicated, and often inaccurate. For
these reasons, much attention has been given to developing the simpler but more reliable
methods in which a soil borer or geotome is used. The latter is simple, inexpensive, and
accurate. It can be carried easily upon daily trips or upon longer reconnaissances, and is
always ready for instant use. In the determination of physiological water-content, it is
practically indispensable. Indeed, the readiness with which geotome determinations of
water-content can be made should hasten the universal recognition of the fact that it is
the available, and not the total amount of water in the soil, which determines the effect
upon the plant.
Geotome Methods
41. The geotome. In its simplest form,
the geotome is merely a stout iron tube
with a sharp cutting edge at one end and a
firmly attached handle at the other. The
length is variable and is primarily
determined by the location of the active
root surface of the plant. In xerophytic
habitats, generally a longer tube is
necessary than in mesophytic ones. The
bore is largely determined by the
character of the soil; for example, a larger
one is necessary for gravel than for loam.
Tubes of small bore also tend to pack the
soil below them, and to give a
correspondingly incomplete core. The
best results have been obtained with
geotomes of ½–1 inch tube. Each
geotome has a removable rod, flattened
into a disk at one end, and bent at the
other, for forcing out the core after it has
been cut from the soil. Sets of geotomes Fig. 1. Geotomes and soil can.
have been made in lengths of 5, 10, 12,
15, 20, and 25 inches. The 12– and 15–
inch forms have been commonly used for herbaceous formations and layers. They are
Page 54
marked in inches so that a sample of any lesser depth may be readily taken. Such a
device is very necessary for gravel soils and in mountain regions, where the subsoil of
rock lies close to the surface.
42. Soil borers. There is a large variety of soil borers to choose from, but none have
been found as simple and satisfactory for relatively shallow readings as the geotome just
described. For deep-rooted plants, many xerophytes, shrubs, and trees, borers of the
auger type are necessary. These are large and heavy, and of necessity slow in operation.
They can not well be carried in an ordinary outfit of instruments, and the size of the soil
sample itself precludes the use of such instruments far from the base station, except on
trips made expressly for obtaining samples from deep-seated layers. For depths from two
to eight feet, the Fraenkel borer is perhaps the most satisfactory, except for the coarser
gravels: it costs $14 or $20 according to the length. For greater depths, or when a larger
core is desirable, the Bausch & Lomb borer, number 16536, which costs $5.25, should be
made use of. This is a ponderous affair and can be employed only on special occasions.
On account of the size of samples obtained by these borers, it is usually most satisfactory
to take a small sample from the core at different depths. Frequently, indeed, a hand
trowel may be readily used to obtain a good sample at a particular depth.
43. Taking samples of soil. In obtaining soil samples, the usual practice is to remove
the air-dried surface, noting its depth, and to sink the geotome with a slow, gentle, boring
movement, in order to avoid packing the soil. This difficulty is further obviated by deep
notches with sharp, beveled edges which are cut at the lower end. In obtaining a fifteen-
inch core, there is also less compression if it be cut five inches at a time. Repeated tests
have shown, however, that the single compressed sample is practically as trustworthy as
the one made in sections. The water-content of the former constantly fell within .5 per
cent of that of the latter, and both varied less than 1 per cent from the dug sample used as
a check. As soon as dug, the core is pressed out of the geotome by the plunger directly
into an air-tight soil can. Bottles may be used as containers, but tin cans are lighter and
more durable. Aluminum cans have been devised for this purpose, but on account of the
expense, “Antikamnia” cans have been used instead. These are tested, and those that are
not water-tight are rejected, although it has been found that, even in these, ordinary soils
do not lose an appreciable amount of water in twenty-four hours. The lid should be
screwed on as quickly as possible, and, as an added precaution, the cans are kept in a
close case until they have been weighed. The cans are numbered consecutively on both
lid and side in such a way that the number may be read at a glance. The numbers are
painted, as a label wears off too rapidly, and scratched numbers are not quickly
discerned.
44. Weighing. Although soil samples have been kept in tight cans outside of cases for
several days without losing a milligram of moisture, the safest plan is to make it a rule to
weigh cans as quickly as possible after bringing them in from the field. Moreover, when
delicate balances are available, it is a good practice to weigh to the milligram. At remote
bases, however, and particularly in the field, and on reconnaissance, where delicate,
expensive instruments are out of place, coarser balances, which weigh accurately to one
centigram, give satisfactory results. The study of efficient water-content values has
device is very necessary for gravel soils and in mountain regions, where the subsoil of
rock lies close to the surface.
42. Soil borers. There is a large variety of soil borers to choose from, but none have
been found as simple and satisfactory for relatively shallow readings as the geotome just
described. For deep-rooted plants, many xerophytes, shrubs, and trees, borers of the
auger type are necessary. These are large and heavy, and of necessity slow in operation.
They can not well be carried in an ordinary outfit of instruments, and the size of the soil
sample itself precludes the use of such instruments far from the base station, except on
trips made expressly for obtaining samples from deep-seated layers. For depths from two
to eight feet, the Fraenkel borer is perhaps the most satisfactory, except for the coarser
gravels: it costs $14 or $20 according to the length. For greater depths, or when a larger
core is desirable, the Bausch & Lomb borer, number 16536, which costs $5.25, should be
made use of. This is a ponderous affair and can be employed only on special occasions.
On account of the size of samples obtained by these borers, it is usually most satisfactory
to take a small sample from the core at different depths. Frequently, indeed, a hand
trowel may be readily used to obtain a good sample at a particular depth.
43. Taking samples of soil. In obtaining soil samples, the usual practice is to remove
the air-dried surface, noting its depth, and to sink the geotome with a slow, gentle, boring
movement, in order to avoid packing the soil. This difficulty is further obviated by deep
notches with sharp, beveled edges which are cut at the lower end. In obtaining a fifteen-
inch core, there is also less compression if it be cut five inches at a time. Repeated tests
have shown, however, that the single compressed sample is practically as trustworthy as
the one made in sections. The water-content of the former constantly fell within .5 per
cent of that of the latter, and both varied less than 1 per cent from the dug sample used as
a check. As soon as dug, the core is pressed out of the geotome by the plunger directly
into an air-tight soil can. Bottles may be used as containers, but tin cans are lighter and
more durable. Aluminum cans have been devised for this purpose, but on account of the
expense, “Antikamnia” cans have been used instead. These are tested, and those that are
not water-tight are rejected, although it has been found that, even in these, ordinary soils
do not lose an appreciable amount of water in twenty-four hours. The lid should be
screwed on as quickly as possible, and, as an added precaution, the cans are kept in a
close case until they have been weighed. The cans are numbered consecutively on both
lid and side in such a way that the number may be read at a glance. The numbers are
painted, as a label wears off too rapidly, and scratched numbers are not quickly
discerned.
44. Weighing. Although soil samples have been kept in tight cans outside of cases for
several days without losing a milligram of moisture, the safest plan is to make it a rule to
weigh cans as quickly as possible after bringing them in from the field. Moreover, when
delicate balances are available, it is a good practice to weigh to the milligram. At remote
bases, however, and particularly in the field, and on reconnaissance, where delicate,
expensive instruments are out of place, coarser balances, which weigh accurately to one
centigram, give satisfactory results. The study of efficient water-content values has
Page 55
already gone far enough to indicate that
differences less than 1 per cent are
negligible. Indeed, the soil variation in
a single square meter is often as great
as this. The greatest difference possible
in the third place, i. e., that of 9
milligrams, does not produce a
difference of .1 of 1 per cent in the
water-content value. In consequence,
such strong portable balances as
Bausch & Lomb 12308 ($2), which can
be carried anywhere, give entirely
reliable results. The best procedure is
to weigh the soil with the can. Turning
the soil out upon the pan or upon paper
obviates one weighing, but there is
always some slight loss, and the
chances of serious mishap are many.
After weighing, the sample is dried as
rapidly as possible in a water bath or
oven. At a temperature of 100° C. this
is accomplished ordinarily in twenty-
four hours; the most tenacious clays
require a longer time, or a higher
temperature. High temperatures should Fig. 3. American soil
be avoided, however, for soils that borer.
contain much leaf mould or other
organic matter,
in order that this
may not be
destroyed. When
it is necessary on
trips, soil
samples can be
dried in the sun
or even in the
air. This usually
takes several
days, however,
and a test
weighing is
generally
required before
differences less than 1 per cent are
negligible. Indeed, the soil variation in
a single square meter is often as great
as this. The greatest difference possible
in the third place, i. e., that of 9
milligrams, does not produce a
difference of .1 of 1 per cent in the
water-content value. In consequence,
such strong portable balances as
Bausch & Lomb 12308 ($2), which can
be carried anywhere, give entirely
reliable results. The best procedure is
to weigh the soil with the can. Turning
the soil out upon the pan or upon paper
obviates one weighing, but there is
always some slight loss, and the
chances of serious mishap are many.
After weighing, the sample is dried as
rapidly as possible in a water bath or
oven. At a temperature of 100° C. this
is accomplished ordinarily in twenty-
four hours; the most tenacious clays
require a longer time, or a higher
temperature. High temperatures should Fig. 3. American soil
be avoided, however, for soils that borer.
contain much leaf mould or other
organic matter,
in order that this
may not be
destroyed. When
it is necessary on
trips, soil
samples can be
dried in the sun
or even in the
air. This usually
takes several
days, however,
and a test
weighing is
generally
required before
Page 56
one can be Fig. 4. Field balance.
certain that the
moisture is
entirely gone. The weighing of the dried soil is made as before,
and the can is carefully brushed out and weighed. The weight of
aluminum cans may be determined once for all, but with painted
cans it has been the practice to weigh them each time.
45. Computation. The most direct method of expressing the
water-content is by per cents figured upon the moist soil as a
basis. The ideal way would be to determine the actual amount of
water per unit volume, but as this would necessitate weighing
one unit volume at least in every habitat studied, as a
preliminary step, it is not practicable. The actual process of
computation is extremely simple. The weight of the dried
sample, w1, is subtracted from the weight of the original sample,
w, and the weight of the can, w2, is likewise subtracted from w.
The first result is then divided by the second, giving the per cent
of water, or the physical water-content. The formula is: (w −
w1)/(w − w2) = W. The result is expressed preferably in grams
per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per
cent of water-content may readily be figured on the basis of dry
or moist soil.
46. Time and location of readings. Owing to the daily
change in the amount of soil water due to evaporation, gravity,
and rainfall, an isolated determination of water-content has very
little value. It is a primary requisite that a basis for comparison
be established by making (1) a series of readings in the same
place, (2) a series at practically the same time in a number of
different places or habitats, or (3) by combining the two
methods, and following the daily changes of a series of stations
throughout an entire season, or at least for a period sufficient to
determine the approximate maximum and minimum. The last
procedure can hardly be carried out except at a base station, but
here it is practically indispensable. It has been followed both at
Lincoln and at Minnehaha, resulting in a basal series for each
place that is of the greatest importance. When such a base
already exists, or, better, while it is being established, scattered
readings may be used somewhat profitably. As a practical
working rule, however, it is most convenient and satisfactory to
make all determinations consecutively, i. e., in a series of
stations or of successive days. Under ordinary conditions, the
time of day at which a particular sample is taken is of little
importance, as the variation during a day is usually slight. This
certain that the
moisture is
entirely gone. The weighing of the dried soil is made as before,
and the can is carefully brushed out and weighed. The weight of
aluminum cans may be determined once for all, but with painted
cans it has been the practice to weigh them each time.
45. Computation. The most direct method of expressing the
water-content is by per cents figured upon the moist soil as a
basis. The ideal way would be to determine the actual amount of
water per unit volume, but as this would necessitate weighing
one unit volume at least in every habitat studied, as a
preliminary step, it is not practicable. The actual process of
computation is extremely simple. The weight of the dried
sample, w1, is subtracted from the weight of the original sample,
w, and the weight of the can, w2, is likewise subtracted from w.
The first result is then divided by the second, giving the per cent
of water, or the physical water-content. The formula is: (w −
w1)/(w − w2) = W. The result is expressed preferably in grams
per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per
cent of water-content may readily be figured on the basis of dry
or moist soil.
46. Time and location of readings. Owing to the daily
change in the amount of soil water due to evaporation, gravity,
and rainfall, an isolated determination of water-content has very
little value. It is a primary requisite that a basis for comparison
be established by making (1) a series of readings in the same
place, (2) a series at practically the same time in a number of
different places or habitats, or (3) by combining the two
methods, and following the daily changes of a series of stations
throughout an entire season, or at least for a period sufficient to
determine the approximate maximum and minimum. The last
procedure can hardly be carried out except at a base station, but
here it is practically indispensable. It has been followed both at
Lincoln and at Minnehaha, resulting in a basal series for each
place that is of the greatest importance. When such a base
already exists, or, better, while it is being established, scattered
readings may be used somewhat profitably. As a practical
working rule, however, it is most convenient and satisfactory to
make all determinations consecutively, i. e., in a series of
stations or of successive days. Under ordinary conditions, the
time of day at which a particular sample is taken is of little
importance, as the variation during a day is usually slight. This
Page 57
Fig. 2. Fraenkel soil does not hold for exposed wet soils, and especially for soils
borer. which have just been wetted by rains. In all comparative series,
however, the samples should be taken at the same hour
whenever possible. This is particularly necessary when it is
desired to ascertain the daily decrease of water-content in the same spot. In the case of a
series of stations, these should be read always in the same order, at the same time of day,
and as rapidly as possible. When a daily station series is being run, i. e., a series by days
and stations both, the daily reading for each place should fall at the same time. While
there are certain advantages in making readings either early or late in the day, they may
be made at any time if the above precautions are followed.
47. Location of readings. Samples should invariably be taken in spots which are both
typical and normal, especially when they are to be used as representative of a particular
area or habitat. A slight change in slope, soil composition, in the amount of dead or living
cover, etc., will produce considerable change in the amount of water present. Where
habitat and formation are uniform, fewer precautions are necessary. This is a rare
circumstance, and as a rule determinations must be made wherever appreciable
differences are in evidence. The problem is simpler when readings are taken with
reference to the structure or modifications of a particular species, but even here, check
readings in several places are of great value. The variation of water in a spot apparently
uniform has been found to be slight in the prairies and the mountains. In taking three
samples in spots a few inches to several feet apart, the difference in the amount of water
has rarely exceeded 1 per cent, which is practically negligible. Gardner[2] found that 16
samples taken to a depth of 3 inches, in as many different portions of a carefully
prepared, denuded soil plot, showed a variation of 7½ per cent. This is partially
explained by the shallowness of the samples, but even then the results of the two
investigations are in serious conflict and indicate that the question needs especial study.
It should be further pointed out that all readings should be made well within a particular
area, and not near its edge, and that, in the case of large diversified habitats, it is the
consocies and the society which indicate the obvious variations in the structure of the
habitat.
48. Depth of samples. The general rule is that the depth of soil samples is determined
by the layer to which the roots penetrate. The practice is to remove the air-dried surface
in which no roots are found, and to take a sample to the proper depth. This method is
open to some objection, as the actively absorbing root surfaces are often localized. There
is no practical way of taking account of this as yet, except in the case of deep-rooted
xerophytes and woody plants. It is practicable to determine the location of the active root
area of a particular plant and hence the water-content of the soil layer, but in most
formations, roots penetrate to such different depths that a sample which includes the
greater part of the distance concerned is satisfactory. Some knowledge of the soil of a
formation is also necessary, since shallow soils do not require as deep samples as others.
The same is true of shaded soils without reference to their depth, and, in large measure,
of soils supplied with telluric water. In all cases, it is highly desirable to have numerous
control-samples at different depths. The normal cores are 12 or 15 inches; control-
borer. which have just been wetted by rains. In all comparative series,
however, the samples should be taken at the same hour
whenever possible. This is particularly necessary when it is
desired to ascertain the daily decrease of water-content in the same spot. In the case of a
series of stations, these should be read always in the same order, at the same time of day,
and as rapidly as possible. When a daily station series is being run, i. e., a series by days
and stations both, the daily reading for each place should fall at the same time. While
there are certain advantages in making readings either early or late in the day, they may
be made at any time if the above precautions are followed.
47. Location of readings. Samples should invariably be taken in spots which are both
typical and normal, especially when they are to be used as representative of a particular
area or habitat. A slight change in slope, soil composition, in the amount of dead or living
cover, etc., will produce considerable change in the amount of water present. Where
habitat and formation are uniform, fewer precautions are necessary. This is a rare
circumstance, and as a rule determinations must be made wherever appreciable
differences are in evidence. The problem is simpler when readings are taken with
reference to the structure or modifications of a particular species, but even here, check
readings in several places are of great value. The variation of water in a spot apparently
uniform has been found to be slight in the prairies and the mountains. In taking three
samples in spots a few inches to several feet apart, the difference in the amount of water
has rarely exceeded 1 per cent, which is practically negligible. Gardner[2] found that 16
samples taken to a depth of 3 inches, in as many different portions of a carefully
prepared, denuded soil plot, showed a variation of 7½ per cent. This is partially
explained by the shallowness of the samples, but even then the results of the two
investigations are in serious conflict and indicate that the question needs especial study.
It should be further pointed out that all readings should be made well within a particular
area, and not near its edge, and that, in the case of large diversified habitats, it is the
consocies and the society which indicate the obvious variations in the structure of the
habitat.
48. Depth of samples. The general rule is that the depth of soil samples is determined
by the layer to which the roots penetrate. The practice is to remove the air-dried surface
in which no roots are found, and to take a sample to the proper depth. This method is
open to some objection, as the actively absorbing root surfaces are often localized. There
is no practical way of taking account of this as yet, except in the case of deep-rooted
xerophytes and woody plants. It is practicable to determine the location of the active root
area of a particular plant and hence the water-content of the soil layer, but in most
formations, roots penetrate to such different depths that a sample which includes the
greater part of the distance concerned is satisfactory. Some knowledge of the soil of a
formation is also necessary, since shallow soils do not require as deep samples as others.
The same is true of shaded soils without reference to their depth, and, in large measure,
of soils supplied with telluric water. In all cases, it is highly desirable to have numerous
control-samples at different depths. The normal cores are 12 or 15 inches; control-
Page 58
samples are taken every 5 inches to the depth desired, and in some cases 3–inch sections
are made. It has been found a great saving of time to combine these methods. A 5–inch
sample is taken and placed in one can, then a second one, and a third in like manner. In
this way the water-content of each 5–inch layer is determined, and from the combined
weight the total content is readily ascertained.
49. Check and control instruments. A number of instruments throw much light upon
the general relations of soil water. The rain-gauge, or ombrometer, measures the
periodical replenishment of the water supply, and has a direct bearing upon seasonal
variation. The atmometer affords a clue to the daily decrease of water by evaporation,
and thus supplements the rain-gauge. The run-off gauge enables one to establish a direct
connection between water-content and the slope and character of the surface. The amount
and rapidity of absorption are determined by means of a simple instrument termed a
rhoptometer. The gravitation water of a soil is ascertained by a hizometer, and some clue
to the hygroscopic and capillary water may be obtained by an artificial osmotic cell. All
of these are of importance because they serve to explain the water-content of a particular
soil with especial reference to the other factors of the habitat. It is evident that none of
them can actually be used in exact determinations of the amount of water, and they will
be considered under the factors with which, they are more immediately concerned.
Physical and Physiological Water
50. The availability of soil water. The amount of water present in a soil is no real
index to the influence of water-content as a factor of the habitat. All soils contain more
water than can be absorbed by the plants which grow in them. This residual water, which
is not available for use, varies for different soils. It is greatest in the compact soils, such
as clay and loam, and least in the loose ones, as sand and gravel. It differs, but to a much
less degree, from one species to another. A plant of xerophytic tendency is naturally able
to remove more water from the same soil than one of mesophytic or hydrophytic
character. As the species of a particular formation owe their association chiefly to their
common relation to the water-content of the habitat, this difference is of little importance
in the field. In comparing the structure of formations, and especially that of the plants
which are found in them, the need to distinguish the available water from the total
amount is imperative. Thus, water-contents of 15 per cent in gravel and in clay are in no
wise comparable. A coarse gravel containing 15 per cent of water is practically saturated.
The plants which grow upon it are mesophytes of a strong hydrophytic tendency, and
they are able to use 14½ or all but .5 out of the 15 per cent of water. In a compact clay,
only 3½ of the 15 per cent are available, and the plants growing in it are marked
xerophytes. It is evident that a knowledge merely of the physical water-content is
actually misleading in such cases, and this holds true of comparisons of any soils which
differ considerably in texture. After one has determined the physiological water for the
great groups of soils, it is more or less possible to estimate the amounts in the various
types of each. As an analysis is necessary to show how close soils are in texture, this is
are made. It has been found a great saving of time to combine these methods. A 5–inch
sample is taken and placed in one can, then a second one, and a third in like manner. In
this way the water-content of each 5–inch layer is determined, and from the combined
weight the total content is readily ascertained.
49. Check and control instruments. A number of instruments throw much light upon
the general relations of soil water. The rain-gauge, or ombrometer, measures the
periodical replenishment of the water supply, and has a direct bearing upon seasonal
variation. The atmometer affords a clue to the daily decrease of water by evaporation,
and thus supplements the rain-gauge. The run-off gauge enables one to establish a direct
connection between water-content and the slope and character of the surface. The amount
and rapidity of absorption are determined by means of a simple instrument termed a
rhoptometer. The gravitation water of a soil is ascertained by a hizometer, and some clue
to the hygroscopic and capillary water may be obtained by an artificial osmotic cell. All
of these are of importance because they serve to explain the water-content of a particular
soil with especial reference to the other factors of the habitat. It is evident that none of
them can actually be used in exact determinations of the amount of water, and they will
be considered under the factors with which, they are more immediately concerned.
Physical and Physiological Water
50. The availability of soil water. The amount of water present in a soil is no real
index to the influence of water-content as a factor of the habitat. All soils contain more
water than can be absorbed by the plants which grow in them. This residual water, which
is not available for use, varies for different soils. It is greatest in the compact soils, such
as clay and loam, and least in the loose ones, as sand and gravel. It differs, but to a much
less degree, from one species to another. A plant of xerophytic tendency is naturally able
to remove more water from the same soil than one of mesophytic or hydrophytic
character. As the species of a particular formation owe their association chiefly to their
common relation to the water-content of the habitat, this difference is of little importance
in the field. In comparing the structure of formations, and especially that of the plants
which are found in them, the need to distinguish the available water from the total
amount is imperative. Thus, water-contents of 15 per cent in gravel and in clay are in no
wise comparable. A coarse gravel containing 15 per cent of water is practically saturated.
The plants which grow upon it are mesophytes of a strong hydrophytic tendency, and
they are able to use 14½ or all but .5 out of the 15 per cent of water. In a compact clay,
only 3½ of the 15 per cent are available, and the plants growing in it are marked
xerophytes. It is evident that a knowledge merely of the physical water-content is
actually misleading in such cases, and this holds true of comparisons of any soils which
differ considerably in texture. After one has determined the physiological water for the
great groups of soils, it is more or less possible to estimate the amounts in the various
types of each. As an analysis is necessary to show how close soils are in texture, this is
Page 59
little better than a guess, and for accurate work it is indispensable that the available water
be determined for each habitat. Within the same formation, however, after this has once
been carefully ascertained, it is perfectly satisfactory to convert physical water-content
into available by subtracting the non-available water, which under normal conditions in
the field remains practically the same.
The importance of knowing the available water is even greater in those habitats in
which salts, acids, cold, or other factors than the molecular attraction of soil particles
increase the amount of water which the plant can not absorb. Few careful investigations
of such soils have yet been made, and the relation of available to non-available water in
them is almost entirely unknown. It is probable that the phenomena in some of these will
be found to be produced by other factors.
51. Terms. The terms, physiological water-content, and physical water-content, are
awkward and not altogether clear in their application. It is here proposed to replace them
by short words which will refer directly to the availability of the soil water for absorption
by the plant. Accordingly, the total amount of water in the soil is divided into the
available and the non-available water-content. The terms suggested for these are
respectively, holard (ὅλos, whole, ἅpδov, water), chresard (χοῆςις, use), and echard (ἕχω,
to withhold).
52. Chresard determinations under control. The determination of the chresard in
the field is attended with peculiar difficulties. In consequence, the method of obtaining it
under control will first be described. The inquiry may be made with reference to soils in
general or to the soil of a particular formation. In the last case, if the plants used are from
the same formation, the results will have almost the value of a field determination. When
no definite habitat is the subject of investigation, an actual soil, and not an artificial
mixture, should be used, and the plants employed should be mesophytes. The individual
plants are grown from seeds in the proper soils, and are repotted sufficiently often to
keep the roots away from the surface. The last transfer is made to a pot large enough to
permit the plant to become full-grown without crowding the roots. The pot should be
glazed inside and out in order to prevent the escape of moisture. This interferes slightly
with the aeration of the soil, but it will not cause any real difficulty. The plant is watered
in such a way as to make the growth as normal as possible. After it has become well
established, three soil samples are taken in such a manner that they will give the variation
in different parts of the pot. One is taken near the plant, the second midway between the
plant and the edge of the pot, and the third near the edge. The depth is determined by the
size of the pot and the position of the roots. The holard is determined for these in the
usual way, but the result is expressed with reference to 100 grams of dry soil; the average
is taken as representative. The soil is then allowed to dry out slowly, as sudden drouth
will sometimes impair the power of absorption and a plant will wilt although
considerable available water remains. Plants often wilt in the field daily for several
successive hot dry days, and become completely turgid again during the night. If the
drying out takes place slowly, the plant will not recover after it has once begun to wilt.
The proper time to make the second reading is indicated by the pronounced wilting of the
leaves and shoots. Complete wilting occurs, as a rule, only after the younger parts have
be determined for each habitat. Within the same formation, however, after this has once
been carefully ascertained, it is perfectly satisfactory to convert physical water-content
into available by subtracting the non-available water, which under normal conditions in
the field remains practically the same.
The importance of knowing the available water is even greater in those habitats in
which salts, acids, cold, or other factors than the molecular attraction of soil particles
increase the amount of water which the plant can not absorb. Few careful investigations
of such soils have yet been made, and the relation of available to non-available water in
them is almost entirely unknown. It is probable that the phenomena in some of these will
be found to be produced by other factors.
51. Terms. The terms, physiological water-content, and physical water-content, are
awkward and not altogether clear in their application. It is here proposed to replace them
by short words which will refer directly to the availability of the soil water for absorption
by the plant. Accordingly, the total amount of water in the soil is divided into the
available and the non-available water-content. The terms suggested for these are
respectively, holard (ὅλos, whole, ἅpδov, water), chresard (χοῆςις, use), and echard (ἕχω,
to withhold).
52. Chresard determinations under control. The determination of the chresard in
the field is attended with peculiar difficulties. In consequence, the method of obtaining it
under control will first be described. The inquiry may be made with reference to soils in
general or to the soil of a particular formation. In the last case, if the plants used are from
the same formation, the results will have almost the value of a field determination. When
no definite habitat is the subject of investigation, an actual soil, and not an artificial
mixture, should be used, and the plants employed should be mesophytes. The individual
plants are grown from seeds in the proper soils, and are repotted sufficiently often to
keep the roots away from the surface. The last transfer is made to a pot large enough to
permit the plant to become full-grown without crowding the roots. The pot should be
glazed inside and out in order to prevent the escape of moisture. This interferes slightly
with the aeration of the soil, but it will not cause any real difficulty. The plant is watered
in such a way as to make the growth as normal as possible. After it has become well
established, three soil samples are taken in such a manner that they will give the variation
in different parts of the pot. One is taken near the plant, the second midway between the
plant and the edge of the pot, and the third near the edge. The depth is determined by the
size of the pot and the position of the roots. The holard is determined for these in the
usual way, but the result is expressed with reference to 100 grams of dry soil; the average
is taken as representative. The soil is then allowed to dry out slowly, as sudden drouth
will sometimes impair the power of absorption and a plant will wilt although
considerable available water remains. Plants often wilt in the field daily for several
successive hot dry days, and become completely turgid again during the night. If the
drying out takes place slowly, the plant will not recover after it has once begun to wilt.
The proper time to make the second reading is indicated by the pronounced wilting of the
leaves and shoots. Complete wilting occurs, as a rule, only after the younger parts have
Page 60
drawn for some time upon the watery tissues of the stem and root, by which time
evaporation has considerably deceased the water in the soil. It is a well-known fact that
young leaves do not wilt easily, especially in watery or succulent plants. Three samples
are again taken and the average water-content determined as above. This is the non-
available water or the echard. The latter is then computed on the basis of 100 grams of
dry soil, and this result is subtracted from the holard to give the chresard in grams for
each 100 grams of dry weight. The chresard may also be expressed with respect to 100
grams of moist soil. As a final precaution in basal work, it is advisable to determine the
chresard for six individuals of the same species under as nearly the same conditions as
possible. When it is desired, however, to find the average chresard for a particular soil, it
is necessary to employ various species representing diverse phyads and ecads. Such an
investigation is necessarily very complicated, and must be made the subject of special
inquiry.
53. Chresard readings in the field. The especial difficulties which must be overcome
in the field are the exclusion of rain and dew and the cutting off of the capillary water. It
is evident, of course, that experiments of this sort must also be entirely free from outside
disturbance. The choice of an area depends upon the scope of the study. If the chresard is
sought for a particular consocies, the block of soil to be studied should show several
species which are fairly representative. In case the chresard of a certain species is to be
obtained, this species alone need be present, but it should be represented by several
individuals. Check plots are desirable in either event, and at least two or three which are
as nearly uniform as possible should be chosen. The size and depth of the soil block
depends upon the plants concerned. It must be large enough that the roots of the
particular individuals under investigation are not disturbed. There is a limit to the size of
the mass that can be handled readily, and in consequence the test plants must not be too
large or too deeply rooted. The task of cutting out the soil block requires a spade with a
long sharp blade. After ascertaining the spread and depth of the roots, the block is cut so
that a margin of several inches free from the roots concerned is left on the sides and
bottom. If the block is to be lifted out of place, so that the sides are exposed to
evaporation, this allowance should be greater. In some cases, it may be found more
convenient to dig the plant up, place it in a large pot, and put the latter back in the hole.
As a general practice, however, this is much less satisfactory.
After the block has been cut, it may be moved if the soil is sufficiently compact, and
then allowed to dry out in its own formation or elsewhere. The results are most valuable
in the first case, though it is often an advantage to remove blocks cut from shade or wet
formations to dry, sunny stations where they will dry more rapidly. The most satisfactory
and natural method, however, is to leave the block in place, and to prevent the
reestablishment of capillary action by enclosing it within plates. This is accomplished by
slipping thin sheet-iron plates into position along the cut surfaces. The plate for the
bottom should be somewhat wider than the block, and is slipped into place by raising the
block if the soil is not too loose; in the latter event, it is carefully driven in. The side
plates are then pushed down to meet the former. The size of the plates depends upon the
block; in general, plates of 1, 2, and 3 feet square, with the bottom plates a trifle larger,
evaporation has considerably deceased the water in the soil. It is a well-known fact that
young leaves do not wilt easily, especially in watery or succulent plants. Three samples
are again taken and the average water-content determined as above. This is the non-
available water or the echard. The latter is then computed on the basis of 100 grams of
dry soil, and this result is subtracted from the holard to give the chresard in grams for
each 100 grams of dry weight. The chresard may also be expressed with respect to 100
grams of moist soil. As a final precaution in basal work, it is advisable to determine the
chresard for six individuals of the same species under as nearly the same conditions as
possible. When it is desired, however, to find the average chresard for a particular soil, it
is necessary to employ various species representing diverse phyads and ecads. Such an
investigation is necessarily very complicated, and must be made the subject of special
inquiry.
53. Chresard readings in the field. The especial difficulties which must be overcome
in the field are the exclusion of rain and dew and the cutting off of the capillary water. It
is evident, of course, that experiments of this sort must also be entirely free from outside
disturbance. The choice of an area depends upon the scope of the study. If the chresard is
sought for a particular consocies, the block of soil to be studied should show several
species which are fairly representative. In case the chresard of a certain species is to be
obtained, this species alone need be present, but it should be represented by several
individuals. Check plots are desirable in either event, and at least two or three which are
as nearly uniform as possible should be chosen. The size and depth of the soil block
depends upon the plants concerned. It must be large enough that the roots of the
particular individuals under investigation are not disturbed. There is a limit to the size of
the mass that can be handled readily, and in consequence the test plants must not be too
large or too deeply rooted. The task of cutting out the soil block requires a spade with a
long sharp blade. After ascertaining the spread and depth of the roots, the block is cut so
that a margin of several inches free from the roots concerned is left on the sides and
bottom. If the block is to be lifted out of place, so that the sides are exposed to
evaporation, this allowance should be greater. In some cases, it may be found more
convenient to dig the plant up, place it in a large pot, and put the latter back in the hole.
As a general practice, however, this is much less satisfactory.
After the block has been cut, it may be moved if the soil is sufficiently compact, and
then allowed to dry out in its own formation or elsewhere. The results are most valuable
in the first case, though it is often an advantage to remove blocks cut from shade or wet
formations to dry, sunny stations where they will dry more rapidly. The most satisfactory
and natural method, however, is to leave the block in place, and to prevent the
reestablishment of capillary action by enclosing it within plates. This is accomplished by
slipping thin sheet-iron plates into position along the cut surfaces. The plate for the
bottom should be somewhat wider than the block, and is slipped into place by raising the
block if the soil is not too loose; in the latter event, it is carefully driven in. The side
plates are then pushed down to meet the former. The size of the plates depends upon the
block; in general, plates of 1, 2, and 3 feet square, with the bottom plates a trifle larger,
Page 61
are the most serviceable. Access of rain and dew is prevented by an awning of heavy
canvas which projects far enough beyond each side of the block to prevent wetting. The
height will depend of course upon the size of the plants. The awning must be used only
when rain or heavy dew is threatened, as the shade which it produces changes the power
of the plant to draw water from the soil.
The time necessary to cause wilting varies with the habitat and the weather. When the
block is large and in position, two or three weeks are required. This period of drying
incidentally furnishes an excellent opportunity for determining the rate at which the
particular soil loses water. The holard sample is taken daily for several days before the
block is cut out, in order to obtain an average, care being taken of course to avoid a
period of extreme weather. The echard samples are taken as soon as the wilting is
sufficient to indicate that the limit of available water is reached. The air-dry soil above
the roots is first removed. The treatment of the samples and the computation of the
chresard are as previously indicated.
54. Chresard values of different soils. The following table gives the water-content
values of six representative soils. The per cents of holard (at saturation) and of echard are
those determined by Hedgcock[3] with six mesophytes as test plants for each soil. The
chresard has been computed directly from these.
HOLARD ECHARD CHRESARD
Sand 14.3 12.6 .3 .25 14 12.3
Clay 47.4 32.5 9.3 6.3 38.1 26.2
Loess 59.3 37.1 10.1 6.4 49.2 30.7
Loam 64.1 39.1 10.9 6.6 53.2 32.5
Humus 65.3 39.6 11.9 7.2 53.4 32.4
Saline 68.5 40.8 16.2 9.6 52.3 31.2
The first column indicates the per cent based upon the dry weight, the second upon the weight of the moist soil.
While these can not be considered absolute for a particular soil other than the ones
investigated, they are found to correspond somewhat closely to the results obtained for
other soils of the respective groups. For accurate research, the chresard must of course be
ascertained for each formation with respect to its peculiar plants and soil. The influence
of the ecad in more or less determining the echard is also shown by Hedgcock, who
found that floating plants wilt at 25 per cent, amphibious ones at 15–20 per cent,
mesophytes at 6–12 per cent, and mesophytic xerophytes at 3–6 per cent. The echard is
also somewhat higher for shade plants than for heliophytes.
Records and Results
55. The field record. It is superfluous to point out that a definite form for field records
saves much time and prevents many mistakes. The exact form may be left to personal
taste, but there are certain features which are essential. Many of these are evident, while
canvas which projects far enough beyond each side of the block to prevent wetting. The
height will depend of course upon the size of the plants. The awning must be used only
when rain or heavy dew is threatened, as the shade which it produces changes the power
of the plant to draw water from the soil.
The time necessary to cause wilting varies with the habitat and the weather. When the
block is large and in position, two or three weeks are required. This period of drying
incidentally furnishes an excellent opportunity for determining the rate at which the
particular soil loses water. The holard sample is taken daily for several days before the
block is cut out, in order to obtain an average, care being taken of course to avoid a
period of extreme weather. The echard samples are taken as soon as the wilting is
sufficient to indicate that the limit of available water is reached. The air-dry soil above
the roots is first removed. The treatment of the samples and the computation of the
chresard are as previously indicated.
54. Chresard values of different soils. The following table gives the water-content
values of six representative soils. The per cents of holard (at saturation) and of echard are
those determined by Hedgcock[3] with six mesophytes as test plants for each soil. The
chresard has been computed directly from these.
HOLARD ECHARD CHRESARD
Sand 14.3 12.6 .3 .25 14 12.3
Clay 47.4 32.5 9.3 6.3 38.1 26.2
Loess 59.3 37.1 10.1 6.4 49.2 30.7
Loam 64.1 39.1 10.9 6.6 53.2 32.5
Humus 65.3 39.6 11.9 7.2 53.4 32.4
Saline 68.5 40.8 16.2 9.6 52.3 31.2
The first column indicates the per cent based upon the dry weight, the second upon the weight of the moist soil.
While these can not be considered absolute for a particular soil other than the ones
investigated, they are found to correspond somewhat closely to the results obtained for
other soils of the respective groups. For accurate research, the chresard must of course be
ascertained for each formation with respect to its peculiar plants and soil. The influence
of the ecad in more or less determining the echard is also shown by Hedgcock, who
found that floating plants wilt at 25 per cent, amphibious ones at 15–20 per cent,
mesophytes at 6–12 per cent, and mesophytic xerophytes at 3–6 per cent. The echard is
also somewhat higher for shade plants than for heliophytes.
Records and Results
55. The field record. It is superfluous to point out that a definite form for field records
saves much time and prevents many mistakes. The exact form may be left to personal
taste, but there are certain features which are essential. Many of these are evident, while
Page 62
others may seem unnecessary; all, however, have been proved by experience to have
some value in saving time or in preventing confusion. The two fundamental maxims of
field work are that nothing is too trivial to be of importance, and that no detail should be
entrusted to the memory. The field record should contain in unmistakable terms all that
the field has yielded. These statements apply with especial force to water-content, in
many senses the most important of physical factors. The precise character of the record
depends upon the way in which the readings are made, whether scattered or in series. As
the day-station series is of the greatest importance, the record is adapted for it especially,
but it will also serve for all readings. The record is chronological, since this is the only
convenient method for the field. A proper form for a field record of water-content is the
following:
some value in saving time or in preventing confusion. The two fundamental maxims of
field work are that nothing is too trivial to be of importance, and that no detail should be
entrusted to the memory. The field record should contain in unmistakable terms all that
the field has yielded. These statements apply with especial force to water-content, in
many senses the most important of physical factors. The precise character of the record
depends upon the way in which the readings are made, whether scattered or in series. As
the day-station series is of the greatest importance, the record is adapted for it especially,
but it will also serve for all readings. The record is chronological, since this is the only
convenient method for the field. A proper form for a field record of water-content is the
following:
Page 63
HOLARD ECHARD NOTES
Can Date Formation Station Community Soil Sample Weighings Weighings Chresard
No. % % Sky Rainfall
1st 2d Can 1st 2d Can
Spruce Jack
10 2/8/04 Mertensiare Loam 10 58.7 50.1 25.52 2 58.7 53.41 25.52 10 16 Cloudy 0
forest Brook
¼
Spruce Milky mold
17 „ Gentianare 10:2 64.25 57.5 21.35 16 64.25 59.6 21.35 5 11 Cloudy 0
forest Way ¾
gravel
Gravel
40 „ Hiawatha Asterare Gravel 2:10 78.55 74.3 22.85 8 78.55 74.85 22.85 1 7 Cloudy 0
slide.
A general designation of the soil composition is a material aid, especially where there
is a difference in the core. For example, in a mountain forest or meadow, the upper layer
will usually be mold, the lower sand or gravel. A careful estimate of the relation between
the two throws much light upon the chresard. Under “sample” the number taken to reach
the desired depth, if more than one, is indicated by placing the number before the depth,
thus 2:10. When two or more full cores are included in the same sample for a check, the
order is 10:2. It has already been shown, however, that these precautions are not
necessary for ordinary purposes. In computing the holard and echard, there is no need to
show the figuring, if the process is checked and then proved. Notes upon sky conditions
aid in explaining the daily decrease in water-content. The amount of rain and the period
during which it falls are of great importance in understanding the fluctuations of the
holard. Under community it is highly desirable to have a list of all the species, but it is
impossible to include this in the table, and a glance at the formation list will show them.
The form indicated above serves for a day-station series, a daily series in one station for
any number of check series in one spot, and for scattered readings. In many cases the
echard will not be determined, but on account of its primary importance, there should be
a space for it, especially since it may be desirable to determine it at some later time.
56. The permanent record. This should be kept by formation, or if the latter exhibits
well-defined associations, the formational record may be divided accordingly. This may
seem an unnecessary expenditure of time, but a slight experience in finding the water-
content values of a particular habitat, when scattered through a chronological field
record, will be convincing. The form of permanent record is the same as for the field,
except that the column for the formation and that for the society are often unnecessary.
57. Sums and means. From the great difficulty of determining the absolute water-
content, and of obtaining a standard of comparison between soils on account of the
varying ratio between bulk and weight, water-content sums are impracticable. For the
same reasons, means of actual water-content are practically impossible, and the mean
water-content must be expressed in per cents. Daily readings are not essential to a
satisfactory mean. In fact, a single reading at each extreme enables one to approximate
the real mean very closely; thus, the average of 26 readings in the prairie formation is 18
per cent. The extremes are 5 per cent and 28 per cent, and their average 16.5 per cent. A
few readings properly scattered through moist and dry periods will give a reliable mean,
as will also a series of daily readings from one heavy rain through a long dry period. The
one difficulty with the last method is that such periods can not well be determined
beforehand. Means permit ready comparison between habitats, but in connecting the
modifications of a species with water-content as a cause, the extremes are significant as
Can Date Formation Station Community Soil Sample Weighings Weighings Chresard
No. % % Sky Rainfall
1st 2d Can 1st 2d Can
Spruce Jack
10 2/8/04 Mertensiare Loam 10 58.7 50.1 25.52 2 58.7 53.41 25.52 10 16 Cloudy 0
forest Brook
¼
Spruce Milky mold
17 „ Gentianare 10:2 64.25 57.5 21.35 16 64.25 59.6 21.35 5 11 Cloudy 0
forest Way ¾
gravel
Gravel
40 „ Hiawatha Asterare Gravel 2:10 78.55 74.3 22.85 8 78.55 74.85 22.85 1 7 Cloudy 0
slide.
A general designation of the soil composition is a material aid, especially where there
is a difference in the core. For example, in a mountain forest or meadow, the upper layer
will usually be mold, the lower sand or gravel. A careful estimate of the relation between
the two throws much light upon the chresard. Under “sample” the number taken to reach
the desired depth, if more than one, is indicated by placing the number before the depth,
thus 2:10. When two or more full cores are included in the same sample for a check, the
order is 10:2. It has already been shown, however, that these precautions are not
necessary for ordinary purposes. In computing the holard and echard, there is no need to
show the figuring, if the process is checked and then proved. Notes upon sky conditions
aid in explaining the daily decrease in water-content. The amount of rain and the period
during which it falls are of great importance in understanding the fluctuations of the
holard. Under community it is highly desirable to have a list of all the species, but it is
impossible to include this in the table, and a glance at the formation list will show them.
The form indicated above serves for a day-station series, a daily series in one station for
any number of check series in one spot, and for scattered readings. In many cases the
echard will not be determined, but on account of its primary importance, there should be
a space for it, especially since it may be desirable to determine it at some later time.
56. The permanent record. This should be kept by formation, or if the latter exhibits
well-defined associations, the formational record may be divided accordingly. This may
seem an unnecessary expenditure of time, but a slight experience in finding the water-
content values of a particular habitat, when scattered through a chronological field
record, will be convincing. The form of permanent record is the same as for the field,
except that the column for the formation and that for the society are often unnecessary.
57. Sums and means. From the great difficulty of determining the absolute water-
content, and of obtaining a standard of comparison between soils on account of the
varying ratio between bulk and weight, water-content sums are impracticable. For the
same reasons, means of actual water-content are practically impossible, and the mean
water-content must be expressed in per cents. Daily readings are not essential to a
satisfactory mean. In fact, a single reading at each extreme enables one to approximate
the real mean very closely; thus, the average of 26 readings in the prairie formation is 18
per cent. The extremes are 5 per cent and 28 per cent, and their average 16.5 per cent. A
few readings properly scattered through moist and dry periods will give a reliable mean,
as will also a series of daily readings from one heavy rain through a long dry period. The
one difficulty with the last method is that such periods can not well be determined
beforehand. Means permit ready comparison between habitats, but in connecting the
modifications of a species with water-content as a cause, the extremes are significant as
Page 64
indicating the range of conditions. Furthermore, the extremes, i. e., 5 per cent and 28 per
cent, make it possible to approximate the mean, 18 per cent, while the latter gives little or
no clue to the extremes. It is hardly necessary to state that means and extremes should be
determined for a certain habitat, or particular area of it, and that the results may be
expressed with reference to holard and chresard.
58. Curves. The value of graphic methods and the details of plotting curves are
reserved for a particular section. It will suffice in this place to indicate the water-content
curves that are of especial value. Simple curves are made with regard to time, place, or
depth. The day curve shows the fluctuations of the water-content of one station from day
to day or from time to time. The station curve indicates the variation in water from
station to station, while the depth curve represents the different values at various depths
in the same station. These may be combined on the same sheet in such a way that the
station curves of each day may be compared directly. Similar combinations may be used
for comparing the day curves, or the depth curves of different stations, but these are of
less importance. A combination of curves which is of the greatest value is one which
admits of direct comparison between the station curves of saturation, holard, chresard,
and echard.
cent, make it possible to approximate the mean, 18 per cent, while the latter gives little or
no clue to the extremes. It is hardly necessary to state that means and extremes should be
determined for a certain habitat, or particular area of it, and that the results may be
expressed with reference to holard and chresard.
58. Curves. The value of graphic methods and the details of plotting curves are
reserved for a particular section. It will suffice in this place to indicate the water-content
curves that are of especial value. Simple curves are made with regard to time, place, or
depth. The day curve shows the fluctuations of the water-content of one station from day
to day or from time to time. The station curve indicates the variation in water from
station to station, while the depth curve represents the different values at various depths
in the same station. These may be combined on the same sheet in such a way that the
station curves of each day may be compared directly. Similar combinations may be used
for comparing the day curves, or the depth curves of different stations, but these are of
less importance. A combination of curves which is of the greatest value is one which
admits of direct comparison between the station curves of saturation, holard, chresard,
and echard.
Page 65
HUMIDITY
59. Instruments. As a direct factor, humidity is intimately connected with water-
content in determining the structure and distribution of plants. The one is in control of
water loss; the other regulates water supply. Humidity as a climatic factor undergoes
greater fluctuation in the same habitat, and the efficient difference is correspondingly
greater. Accordingly, simple instruments are less valuable than automatic ones, since a
continuous record is essential to a proper understanding of the real influence of humidity.
As is the rule, however, the use of simple instruments, when they can be referred to an
ecographic basis, greatly extends the field which can be studied. In investigation, both
psychrometer and psychrograph have their proper place. In the consideration of simple
instruments for obtaining humidity values, an arbitrary distinction is made between
psychrometers and hygrometers. The former consist of a wet and a dry bulb
thermometer, while the latter make use of a hygroscopic awn, hair, or other object.
Psychrometers
60. Kinds. There are three kinds of psychrometer, the sling, the cog, and the
stationary. All consist of a wet bulb and a dry bulb thermometer set in a case; the first
two are designed to be moved or whirled in the air. The same principle is applied in each,
viz., that evaporation produces a decrease in temperature proportional to the amount of
moisture in the air. The dry bulb thermometer is an ordinary thermometer, while the wet
bulb is covered with a cloth that can be moistened. The former indicates the normal
temperature of the air, the latter gives the reduced temperature due to evaporation. The
relative humidity of the air is ascertained by means of the proper tables, from two terms,
i. e., the air temperature and the amount of reduction shown by the wet bulb. The sling
and the cog psychrometers alone are in general use. The stationary form has been found
to be unreliable, because the moisture, as it evaporates from the wet bulb, is not
removed, and, in consequence, hinders evaporation to the proper degree.
61. The sling psychrometer. The standard form of this is shown in the illustration,
and is the one used by the Weather Bureau. This instrument can be obtained from H. J.
Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at
a cost of $5. It consists of a metal frame to which are firmly attached two accurately
standardized thermometers, reading usually from –30° to 130°. The frame is attached at
the uppermost end to a handle in such fashion that it swings freely. The wet bulb
thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth
for the wet bulb should be always of the same texture and quality; the standard used by
the Weather Bureau can be obtained from the instrument makers. A slight difference in
texture makes no appreciable error, but the results obtained with different instruments
and by different observers will be more trustworthy and comparable if the same cloth be
59. Instruments. As a direct factor, humidity is intimately connected with water-
content in determining the structure and distribution of plants. The one is in control of
water loss; the other regulates water supply. Humidity as a climatic factor undergoes
greater fluctuation in the same habitat, and the efficient difference is correspondingly
greater. Accordingly, simple instruments are less valuable than automatic ones, since a
continuous record is essential to a proper understanding of the real influence of humidity.
As is the rule, however, the use of simple instruments, when they can be referred to an
ecographic basis, greatly extends the field which can be studied. In investigation, both
psychrometer and psychrograph have their proper place. In the consideration of simple
instruments for obtaining humidity values, an arbitrary distinction is made between
psychrometers and hygrometers. The former consist of a wet and a dry bulb
thermometer, while the latter make use of a hygroscopic awn, hair, or other object.
Psychrometers
60. Kinds. There are three kinds of psychrometer, the sling, the cog, and the
stationary. All consist of a wet bulb and a dry bulb thermometer set in a case; the first
two are designed to be moved or whirled in the air. The same principle is applied in each,
viz., that evaporation produces a decrease in temperature proportional to the amount of
moisture in the air. The dry bulb thermometer is an ordinary thermometer, while the wet
bulb is covered with a cloth that can be moistened. The former indicates the normal
temperature of the air, the latter gives the reduced temperature due to evaporation. The
relative humidity of the air is ascertained by means of the proper tables, from two terms,
i. e., the air temperature and the amount of reduction shown by the wet bulb. The sling
and the cog psychrometers alone are in general use. The stationary form has been found
to be unreliable, because the moisture, as it evaporates from the wet bulb, is not
removed, and, in consequence, hinders evaporation to the proper degree.
61. The sling psychrometer. The standard form of this is shown in the illustration,
and is the one used by the Weather Bureau. This instrument can be obtained from H. J.
Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at
a cost of $5. It consists of a metal frame to which are firmly attached two accurately
standardized thermometers, reading usually from –30° to 130°. The frame is attached at
the uppermost end to a handle in such fashion that it swings freely. The wet bulb
thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth
for the wet bulb should be always of the same texture and quality; the standard used by
the Weather Bureau can be obtained from the instrument makers. A slight difference in
texture makes no appreciable error, but the results obtained with different instruments
and by different observers will be more trustworthy and comparable if the same cloth be
Page 66
used in all cases. The jacket for the wet
bulb may be sewed in the form of a close-
fitting bag, which soon shrinks and clings
tightly. It may be made in the field by
wrapping the cloth so that the edges just
overlap, and tying it tightly above and
below the bulb. In either case, a single
layer of cloth alone must be used. The
cloth becomes soiled or thin after a few
months’ constant use and should be
replaced. It is a wise precaution to carry a
small piece of psychrometer cloth in the
field outfit.
62. Readings. All observations should
be made facing the wind, and the
observer should move one or two steps
during the reading to prevent the
possibility of error. The cloth of the wet
bulb is moistened with water by means of
a brush, or, much better, it is dipped
directly into a bottle of water. Distilled
water is preferable, as it contains no
dissolved material to accumulate in the
cloth. Tap-water and the water of streams
may be used without appreciable error, if
the cloth is changed somewhat more
frequently. The temperature of the water
is practically negligible under ordinary
conditions. Readings can be made more
quickly, however, when the temperature is
not too far from that of the air. The
psychrometer is held firmly and swung
rapidly through the air when the space is
not too confined. Where there is danger
of breakage, it is swung back and forth
through a short arc, pendulum-fashion. As
the reading must be made when the
mercury of the wet bulb reaches the
lowest point, the instrument is stopped
from time to time and the position of the
column noted. The lowest point is often
indicated by the tendency of the mercury
to remain stationary; as a rule it can be
bulb may be sewed in the form of a close-
fitting bag, which soon shrinks and clings
tightly. It may be made in the field by
wrapping the cloth so that the edges just
overlap, and tying it tightly above and
below the bulb. In either case, a single
layer of cloth alone must be used. The
cloth becomes soiled or thin after a few
months’ constant use and should be
replaced. It is a wise precaution to carry a
small piece of psychrometer cloth in the
field outfit.
62. Readings. All observations should
be made facing the wind, and the
observer should move one or two steps
during the reading to prevent the
possibility of error. The cloth of the wet
bulb is moistened with water by means of
a brush, or, much better, it is dipped
directly into a bottle of water. Distilled
water is preferable, as it contains no
dissolved material to accumulate in the
cloth. Tap-water and the water of streams
may be used without appreciable error, if
the cloth is changed somewhat more
frequently. The temperature of the water
is practically negligible under ordinary
conditions. Readings can be made more
quickly, however, when the temperature is
not too far from that of the air. The
psychrometer is held firmly and swung
rapidly through the air when the space is
not too confined. Where there is danger
of breakage, it is swung back and forth
through a short arc, pendulum-fashion. As
the reading must be made when the
mercury of the wet bulb reaches the
lowest point, the instrument is stopped
from time to time and the position of the
column noted. The lowest point is often
indicated by the tendency of the mercury
to remain stationary; as a rule it can be
Page 67
noted with certainty when the next glance
shows a rise in the column. In following
the movement, and especially in noting
the final reading, great care must be taken
to make the latter before the mercury
begins to rise. For this reason it is
desirable to shade the psychrometer with
the body when looking at it, and to take
pains not to breathe upon the bulbs nor to
bring them too near the body. At the
moment when the wet bulb registers the
lowest point, the dry bulb should be read
and the results recorded.
63. Cog psychrometer. This
instrument, commonly called the “egg-
beater” psychrometer, has been devised to
obviate certain disadvantages of the sling
psychrometer in field work, and has
entirely supplanted the latter in the
writer’s own studies. It is smaller, more
compact, and the danger of breaking in
carriage or in use is almost nil. It has the
great advantage of making it possible to
take readings in a layer of air less than
two inches in thickness, and in any
position. Fairly accurate results can even
be obtained from transpiring leaves. The
instrument can readily be made by a good
mechanic, at a cost for materials of $1.75,
which is less than half the price for the
sling form. A single drawback exists in
the use of short, Centigrade
thermometers, inasmuch as tables of
relative humidity are usually expressed in
Fahrenheit. It is a simple matter, however,
to convert Centigrade degrees into
Fahrenheit, mentally, or the difficulty
may be avoided by the conversion table
shown on page 47, or by constructing a
Centigrade series of humidity tables. The
fact that the wet and dry bulbs revolve in
the same path has raised a doubt
concerning the accuracy of the results
shows a rise in the column. In following
the movement, and especially in noting
the final reading, great care must be taken
to make the latter before the mercury
begins to rise. For this reason it is
desirable to shade the psychrometer with
the body when looking at it, and to take
pains not to breathe upon the bulbs nor to
bring them too near the body. At the
moment when the wet bulb registers the
lowest point, the dry bulb should be read
and the results recorded.
63. Cog psychrometer. This
instrument, commonly called the “egg-
beater” psychrometer, has been devised to
obviate certain disadvantages of the sling
psychrometer in field work, and has
entirely supplanted the latter in the
writer’s own studies. It is smaller, more
compact, and the danger of breaking in
carriage or in use is almost nil. It has the
great advantage of making it possible to
take readings in a layer of air less than
two inches in thickness, and in any
position. Fairly accurate results can even
be obtained from transpiring leaves. The
instrument can readily be made by a good
mechanic, at a cost for materials of $1.75,
which is less than half the price for the
sling form. A single drawback exists in
the use of short, Centigrade
thermometers, inasmuch as tables of
relative humidity are usually expressed in
Fahrenheit. It is a simple matter, however,
to convert Centigrade degrees into
Fahrenheit, mentally, or the difficulty
may be avoided by the conversion table
shown on page 47, or by constructing a
Centigrade series of humidity tables. The
fact that the wet and dry bulbs revolve in
the same path has raised a doubt
concerning the accuracy of the results
Page 68
Fig. 5. Sling psychrometer. obtained with this instrument. Repeated
comparisons with the sling psychrometer
have not only removed this doubt
completely, but have also proved that the
standardization of the thermometers has
been efficient.
64. Construction and use. A
convenient form of egg-beater is the Lyon
(Albany, New York), in which the
revolving plates can be readily removed,
leaving the axis and the frame. The
thermometers used are of the short
Centigrade type. They are 4½ inches long
and read from –5° to 50°. Eimer and
Amend, 205 Third Ave., New York city,
furnish them at 75 cents each. The
thermometers are carefully standardized
and compared, and then grouped in pairs
that read together. Each pair is used to
construct a particular psychrometer. Each
thermometer is strongly wired to one side
of the frame, pieces of felt being used to
protect the tube and increase the contact.
The frame is also bent at the base angles Fig. 6. Cog psychrometer.
to permit free circulation of air about the
thermometer bulbs. The bulb of one thermometer is covered with the proper cloth, and
the psychrometer is finished. Since the frame revolves with the thermometers, it is
necessary to pour the water on the wet bulb, or to employ a pipette or brush. The
thermometer bulbs are placed in the layer to be studied, and the frame rotated at an even
rate and with moderate rapidity. The observation is further made as in the case of the
sling psychrometer. As the circle of rotation is less than three inches in diameter, and the
layer less than an inch, in place of nearly three feet for the sling form, the instrument
should not be moved at all for extremely localized readings, but it must be moved
considerably, a foot or more, if it is desirable to obtain a more general reading.
65. Hygrometers. While there are instruments designed to indicate the humidity by
means of a hygroscopic substance, not one of them seems to be of sufficient accuracy for
use in ecological study. The difficulty is that the hygroscopic reaction is inconstant,
rather than that the instruments are not sufficiently sensitive. A number of hygrometers
have been tested, and in all the error has been found to be great, varying usually from
10–20 per cent. In the middle of the scale they sometimes read more accurately, but
toward either extreme they are very inexact. It seems probable that an accurate
hygrometer can be constructed only after the model of the Draper psychrograph. Its
weight and bulk would make it an impossible instrument for field trips, and the expense
comparisons with the sling psychrometer
have not only removed this doubt
completely, but have also proved that the
standardization of the thermometers has
been efficient.
64. Construction and use. A
convenient form of egg-beater is the Lyon
(Albany, New York), in which the
revolving plates can be readily removed,
leaving the axis and the frame. The
thermometers used are of the short
Centigrade type. They are 4½ inches long
and read from –5° to 50°. Eimer and
Amend, 205 Third Ave., New York city,
furnish them at 75 cents each. The
thermometers are carefully standardized
and compared, and then grouped in pairs
that read together. Each pair is used to
construct a particular psychrometer. Each
thermometer is strongly wired to one side
of the frame, pieces of felt being used to
protect the tube and increase the contact.
The frame is also bent at the base angles Fig. 6. Cog psychrometer.
to permit free circulation of air about the
thermometer bulbs. The bulb of one thermometer is covered with the proper cloth, and
the psychrometer is finished. Since the frame revolves with the thermometers, it is
necessary to pour the water on the wet bulb, or to employ a pipette or brush. The
thermometer bulbs are placed in the layer to be studied, and the frame rotated at an even
rate and with moderate rapidity. The observation is further made as in the case of the
sling psychrometer. As the circle of rotation is less than three inches in diameter, and the
layer less than an inch, in place of nearly three feet for the sling form, the instrument
should not be moved at all for extremely localized readings, but it must be moved
considerably, a foot or more, if it is desirable to obtain a more general reading.
65. Hygrometers. While there are instruments designed to indicate the humidity by
means of a hygroscopic substance, not one of them seems to be of sufficient accuracy for
use in ecological study. The difficulty is that the hygroscopic reaction is inconstant,
rather than that the instruments are not sufficiently sensitive. A number of hygrometers
have been tested, and in all the error has been found to be great, varying usually from
10–20 per cent. In the middle of the scale they sometimes read more accurately, but
toward either extreme they are very inexact. It seems probable that an accurate
hygrometer can be constructed only after the model of the Draper psychrograph. Its
weight and bulk would make it an impossible instrument for field trips, and the expense
Page 69
of one would provide a dozen psychrometers. In consequence, it does not seem too
sweeping to say that no hygrometer can furnish trustworthy results. Of simple
instruments for humidity, the psychrometer alone can be trusted to give reliable readings.
Crova’s hygrometer, used by Hesselmann, is not a hygrometer in the sense indicated. As
it is much less convenient to handle and to operate than the cog psychrometer, it is not
necessary to describe it.
Psychrographs
66. The Draper psychrograph. A
year’s trial of the Draper psychrograph in
field and planthouse has left little
question of its accuracy and its great
usefulness. Essentially, it consists of a
band of fine catgut strings, which are
sensitive to changes in the moisture-
content of the air. The variations in the
length of the band are communicated to a
long pointer carrying an inking pen. The
latter traces the record in per cent of
relative humidity on a graduated paper
disk, which is practically the face of an
eight-day clock. The whole is enclosed in
a metal case with a glass front. A glance
at the illustration will show the general
structure of the instrument. Continued
psychrometric tests demonstrate that the
margin of error is well within the efficient
difference for humidity, which is taken to
be 5 per cent. In the field tests of the past
Fig. 7. Draper psychrograph. summer, two psychrographs placed side
by side in the same habitat did not vary 1
per cent from each other. The same
instruments when in different habitats did not deviate more than 1 per cent from the
psychrometric values, except when the air approached saturation. For humidities above
90 per cent, the deviation is considerable, but as these are temporary and incident upon
rainfall, the error is not serious. For humidities varying from 10–85 per cent, the
psychrograph is practically as accurate as the psychrometer. Per cents below 10 are rare,
and no tests have been made for them.
sweeping to say that no hygrometer can furnish trustworthy results. Of simple
instruments for humidity, the psychrometer alone can be trusted to give reliable readings.
Crova’s hygrometer, used by Hesselmann, is not a hygrometer in the sense indicated. As
it is much less convenient to handle and to operate than the cog psychrometer, it is not
necessary to describe it.
Psychrographs
66. The Draper psychrograph. A
year’s trial of the Draper psychrograph in
field and planthouse has left little
question of its accuracy and its great
usefulness. Essentially, it consists of a
band of fine catgut strings, which are
sensitive to changes in the moisture-
content of the air. The variations in the
length of the band are communicated to a
long pointer carrying an inking pen. The
latter traces the record in per cent of
relative humidity on a graduated paper
disk, which is practically the face of an
eight-day clock. The whole is enclosed in
a metal case with a glass front. A glance
at the illustration will show the general
structure of the instrument. Continued
psychrometric tests demonstrate that the
margin of error is well within the efficient
difference for humidity, which is taken to
be 5 per cent. In the field tests of the past
Fig. 7. Draper psychrograph. summer, two psychrographs placed side
by side in the same habitat did not vary 1
per cent from each other. The same
instruments when in different habitats did not deviate more than 1 per cent from the
psychrometric values, except when the air approached saturation. For humidities above
90 per cent, the deviation is considerable, but as these are temporary and incident upon
rainfall, the error is not serious. For humidities varying from 10–85 per cent, the
psychrograph is practically as accurate as the psychrometer. Per cents below 10 are rare,
and no tests have been made for them.
Page 70
Fig. 8. Instrument shelter, showing thermograph and psychrograph in position.
67. Placing the instrument. The psychrograph should be located in a place where the
circulation of the air is typical of the station observed. A satisfactory shelter will screen
the instrument from sun and rain, and at the same time permit the air to pass freely
through the perforations of the metal case. The form shown in figure 8 meets both of
these conditions. A desirable modification is effected by fastening a strip about the cover
of such depth as to prevent the sun’s rays from striking the case except when the sun is
near the horizon. A cross block is fastened on the post of the shelter after being exactly
leveled. The psychrograph rests upon this block, which is three feet above the ground in
order to avoid the influence of radiation. The instrument is held in position by slipping
the eye over a small-headed nail driven obliquely. It does not hang from the latter, but
must rest firmly upon the cross block. The post is set to a depth that prevents oscillation
in the wind, which is liable to obscure the record. In shallow mountain soils stability is
attained by fastening a broad board at the base of the post before setting it. When two or
more psychrographs are established in different habitats, great pains are taken to set them
up in exactly the same way. The shelters are alike, the height above the soil the same, and
the instruments all face the south.
67. Placing the instrument. The psychrograph should be located in a place where the
circulation of the air is typical of the station observed. A satisfactory shelter will screen
the instrument from sun and rain, and at the same time permit the air to pass freely
through the perforations of the metal case. The form shown in figure 8 meets both of
these conditions. A desirable modification is effected by fastening a strip about the cover
of such depth as to prevent the sun’s rays from striking the case except when the sun is
near the horizon. A cross block is fastened on the post of the shelter after being exactly
leveled. The psychrograph rests upon this block, which is three feet above the ground in
order to avoid the influence of radiation. The instrument is held in position by slipping
the eye over a small-headed nail driven obliquely. It does not hang from the latter, but
must rest firmly upon the cross block. The post is set to a depth that prevents oscillation
in the wind, which is liable to obscure the record. In shallow mountain soils stability is
attained by fastening a broad board at the base of the post before setting it. When two or
more psychrographs are established in different habitats, great pains are taken to set them
up in exactly the same way. The shelters are alike, the height above the soil the same, and
the instruments all face the south.
Page 71
68. Regulating and operating the instrument. When two or more psychrographs are
to be used in series, they must be compared with each other in the same spot for several
days until they run exactly together with respect to per cent of humidity and to time.
During this comparison they are checked by the psychrometer and so regulated that they
register the proper humidity. When a single instrument is used alone as the basis to which
simple readings may be referred, all regulating may well be done after the instrument is
in position. This is a simple process; it is accomplished by obtaining the relative
humidity beneath the shelter and at the proper height by a psychrometer. The pen hand is
then moved to the proper line on the disk by means of the screws at its base. These are
reached by removing the lettered glass face. The thumbscrew on the side opposite the
direction in which the pen is to move is released, and the opposite screw simultaneously
tightened, until the pen remains upon the proper line. Experience has proved that the
record sheet should be correctly labeled and dated before being placed on the disk. In the
press of field duties, records labeled after removal are liable to be confused. It is likewise
a great saving of time to write the date of the month in the margin of each segment. Care
is taken to place the sheet on the disk in the same position each time; this can easily be
done by seeing that the sharp point on the disk penetrates the same spot on the paper. A
single drop of ink in the pen will usually give the most satisfactory line. A thin line is
read most accurately. If the pen point is too fine, however, the ink does not flow readily,
and the point should be slightly blunted by means of a file. More often the line is too
broad and the pen must be carefully pointed. Occasionally the pen does not touch the
sheet, and it becomes necessary to bend the hand slightly. This is a frequent difficulty if
the records are folded or wrinkled, and consequently the sheets should always be kept
flat.
69. The weekly visit. Psychrographs must be visited, checked, rewound, and inked
every week. Whenever possible this should be done regularly at a specified day and hour.
This is especially desirable if the same record sheet is used for more than one week.
Time and energy are saved by a fixed order for the various tasks to be done at each visit.
After opening the instrument the disk is removed, and the clock wound, and, if need be,
regulated. The record sheet is replaced, the disk again put on the clock arbor, and the pen
replenished with a drop of ink. A psychrometer reading is made, and the results in terms
of relative humidity noted at the proper place on the disk sheet. If the psychrograph vary
more than 1 per cent, it is adjusted to read accurately. In practice it has been found a
great convenience to keep each record sheet in position for three weeks, and the time
may easily be extended to four. In this event, the pen is carefully cleaned with blotting
paper at each visit, and is then refilled with an ink of different color. To prevent
confusion, the three different colored inks are always used in the same order, red for the
first week, blue for the second, and green for the third. The advantages of this plan are
obvious: fewer records are used and less time is spent in changing them. The records of
several weeks are side by side instead of on separate sheets, and in working over the
season’s results, it is necessary to handle but a third as many sheets.
The Draper psychrograph is made by the Draper Manufacturing Company, 152 Front
St., New York city. The price is $30. A few record sheets and a bottle of red ink are
to be used in series, they must be compared with each other in the same spot for several
days until they run exactly together with respect to per cent of humidity and to time.
During this comparison they are checked by the psychrometer and so regulated that they
register the proper humidity. When a single instrument is used alone as the basis to which
simple readings may be referred, all regulating may well be done after the instrument is
in position. This is a simple process; it is accomplished by obtaining the relative
humidity beneath the shelter and at the proper height by a psychrometer. The pen hand is
then moved to the proper line on the disk by means of the screws at its base. These are
reached by removing the lettered glass face. The thumbscrew on the side opposite the
direction in which the pen is to move is released, and the opposite screw simultaneously
tightened, until the pen remains upon the proper line. Experience has proved that the
record sheet should be correctly labeled and dated before being placed on the disk. In the
press of field duties, records labeled after removal are liable to be confused. It is likewise
a great saving of time to write the date of the month in the margin of each segment. Care
is taken to place the sheet on the disk in the same position each time; this can easily be
done by seeing that the sharp point on the disk penetrates the same spot on the paper. A
single drop of ink in the pen will usually give the most satisfactory line. A thin line is
read most accurately. If the pen point is too fine, however, the ink does not flow readily,
and the point should be slightly blunted by means of a file. More often the line is too
broad and the pen must be carefully pointed. Occasionally the pen does not touch the
sheet, and it becomes necessary to bend the hand slightly. This is a frequent difficulty if
the records are folded or wrinkled, and consequently the sheets should always be kept
flat.
69. The weekly visit. Psychrographs must be visited, checked, rewound, and inked
every week. Whenever possible this should be done regularly at a specified day and hour.
This is especially desirable if the same record sheet is used for more than one week.
Time and energy are saved by a fixed order for the various tasks to be done at each visit.
After opening the instrument the disk is removed, and the clock wound, and, if need be,
regulated. The record sheet is replaced, the disk again put on the clock arbor, and the pen
replenished with a drop of ink. A psychrometer reading is made, and the results in terms
of relative humidity noted at the proper place on the disk sheet. If the psychrograph vary
more than 1 per cent, it is adjusted to read accurately. In practice it has been found a
great convenience to keep each record sheet in position for three weeks, and the time
may easily be extended to four. In this event, the pen is carefully cleaned with blotting
paper at each visit, and is then refilled with an ink of different color. To prevent
confusion, the three different colored inks are always used in the same order, red for the
first week, blue for the second, and green for the third. The advantages of this plan are
obvious: fewer records are used and less time is spent in changing them. The records of
several weeks are side by side instead of on separate sheets, and in working over the
season’s results, it is necessary to handle but a third as many sheets.
The Draper psychrograph is made by the Draper Manufacturing Company, 152 Front
St., New York city. The price is $30. A few record sheets and a bottle of red ink are
Page 72
furnished with it. Additional records can be obtained at 3 cents each. The inks are 25–50
cents per bottle, depending upon the color.
Humidity Readings and Records
70. The time of readings. If simple instruments alone are used for determining
humidity, readings are practically without value unless made simultaneously through
several stations, or successively at one. When it is possible to combine these, and to
make psychrometer readings at different habitats for each hour of the day, or at the same
hour for several days, the series is of very great value. Single readings are unreliable on
account of the hourly and daily variations of humidity, but when these changes are
recorded by a psychrograph, such readings at once become of use, whether made in the
same habitat with the recording instrument or elsewhere. In the latter case, one reading
will tell little about the normal humidity of the habitat, but several make a close estimate
possible. When a series of psychrographs is in use, accurate observations can be made to
advantage anywhere at any time. As a rule, however, it has been found most convenient
to make simple readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as these hours afford much
evidence in regard to the daily range. A good time also is that at which the temperature
maximum occurs each day, but this is movable and in the press of field work can rarely
be taken advantage of. A very fair idea of the daily mean humidity is obtainable by
averaging the readings made at the hours already indicated. The comparison of single
readings with the psychrograph record should not be made at a time when a rapid change
is occurring, as the automatic instrument does not respond immediately. Such a condition
is usually represented by a sudden rain, and is naturally not a satisfactory time for single
readings in any event.
71. Place and height. As stated above, the psychrograph is placed three feet above the
surface of the ground in making readings for the comparison of stations. In low,
herbaceous formations, the instrument is usually placed within a few inches of the soil in
order to record the humidity of the air in which the plants are growing. In forest
formations, the moisture often varies considerably in the different layers. This variation
is easily determined by simultaneous psychrometer readings in the several layers, or, if
occasion warrants, a series of psychrographs may be used. In field work the rule has been
to make observations with the psychrometer at 6 feet, 3 feet, and the surface of the soil,
but the reading at the height of 3 feet is ordinarily sufficient. Humidity varies so easily
that several readings in different parts of one formation are often desirable. In comparing
different formations, the readings should be made in corresponding situations, for
example, in the densest portion of each.
72. Check instruments. Humidity is so readily affected by temperature, wind, and
pressure, that a knowledge of these factors is essential to an understanding of its
fluctuations. Pressure, disregarding daily variation, is taken account of in the tables for
ascertaining relative humidity, and is determined once for all when the altitude of a
station has been carefully established. The temperature is obtained directly from the dry
cents per bottle, depending upon the color.
Humidity Readings and Records
70. The time of readings. If simple instruments alone are used for determining
humidity, readings are practically without value unless made simultaneously through
several stations, or successively at one. When it is possible to combine these, and to
make psychrometer readings at different habitats for each hour of the day, or at the same
hour for several days, the series is of very great value. Single readings are unreliable on
account of the hourly and daily variations of humidity, but when these changes are
recorded by a psychrograph, such readings at once become of use, whether made in the
same habitat with the recording instrument or elsewhere. In the latter case, one reading
will tell little about the normal humidity of the habitat, but several make a close estimate
possible. When a series of psychrographs is in use, accurate observations can be made to
advantage anywhere at any time. As a rule, however, it has been found most convenient
to make simple readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as these hours afford much
evidence in regard to the daily range. A good time also is that at which the temperature
maximum occurs each day, but this is movable and in the press of field work can rarely
be taken advantage of. A very fair idea of the daily mean humidity is obtainable by
averaging the readings made at the hours already indicated. The comparison of single
readings with the psychrograph record should not be made at a time when a rapid change
is occurring, as the automatic instrument does not respond immediately. Such a condition
is usually represented by a sudden rain, and is naturally not a satisfactory time for single
readings in any event.
71. Place and height. As stated above, the psychrograph is placed three feet above the
surface of the ground in making readings for the comparison of stations. In low,
herbaceous formations, the instrument is usually placed within a few inches of the soil in
order to record the humidity of the air in which the plants are growing. In forest
formations, the moisture often varies considerably in the different layers. This variation
is easily determined by simultaneous psychrometer readings in the several layers, or, if
occasion warrants, a series of psychrographs may be used. In field work the rule has been
to make observations with the psychrometer at 6 feet, 3 feet, and the surface of the soil,
but the reading at the height of 3 feet is ordinarily sufficient. Humidity varies so easily
that several readings in different parts of one formation are often desirable. In comparing
different formations, the readings should be made in corresponding situations, for
example, in the densest portion of each.
72. Check instruments. Humidity is so readily affected by temperature, wind, and
pressure, that a knowledge of these factors is essential to an understanding of its
fluctuations. Pressure, disregarding daily variation, is taken account of in the tables for
ascertaining relative humidity, and is determined once for all when the altitude of a
station has been carefully established. The temperature is obtained directly from the dry
Page 73
bulb reading. Its value is fundamental, as
the amount of moisture in a given space is
directly affected by it; like pressure, it
also is taken account of in the formula.
The movement of the air has an
immediate influence upon moisture by
mixing the air of different habitats and
layers. So far as the plant is concerned, it
has practically the effect of increasing or
decreasing the humidity by the removal
of the air above it. Thus, while the
anemometer can furnish no direct
evidence as to the amount of variation, it
is of aid in explaining the reason for it.
Likewise, the rate of evaporation as
indicated by a series of atmometers,
affords a ready method of estimating the
comparative effect of humidity in
different habitats. Potometers and other
instruments for measuring transpiration
throw much light upon humidity values.
Since they are concerned with the
response of the plant to humidity, they are
considered in the following chapter.
73. Humidity tables. To ascertain the
relative humidity, the difference between
the wet and dry bulb readings is obtained.
This, with the dry bulb temperature, is
referred to the tables, where the
corresponding humidity is found. A
variation in temperature has less effect
than a variation in the difference; in
consequence, the dry bulb reading is
expressed in the nearest unit, and the
difference reckoned to the nearest .5. The
humidity varies with the air pressure.
Hence, the altitude must be determined
for the base station, and for all others that
show much change in elevation. Within
the ordinary range of growing-period
temperatures, the effect of pressure is not
great. For all ordinary cases, it suffices to
compute tables for pressures of 30, 29,
the amount of moisture in a given space is
directly affected by it; like pressure, it
also is taken account of in the formula.
The movement of the air has an
immediate influence upon moisture by
mixing the air of different habitats and
layers. So far as the plant is concerned, it
has practically the effect of increasing or
decreasing the humidity by the removal
of the air above it. Thus, while the
anemometer can furnish no direct
evidence as to the amount of variation, it
is of aid in explaining the reason for it.
Likewise, the rate of evaporation as
indicated by a series of atmometers,
affords a ready method of estimating the
comparative effect of humidity in
different habitats. Potometers and other
instruments for measuring transpiration
throw much light upon humidity values.
Since they are concerned with the
response of the plant to humidity, they are
considered in the following chapter.
73. Humidity tables. To ascertain the
relative humidity, the difference between
the wet and dry bulb readings is obtained.
This, with the dry bulb temperature, is
referred to the tables, where the
corresponding humidity is found. A
variation in temperature has less effect
than a variation in the difference; in
consequence, the dry bulb reading is
expressed in the nearest unit, and the
difference reckoned to the nearest .5. The
humidity varies with the air pressure.
Hence, the altitude must be determined
for the base station, and for all others that
show much change in elevation. Within
the ordinary range of growing-period
temperatures, the effect of pressure is not
great. For all ordinary cases, it suffices to
compute tables for pressures of 30, 29,
Page 74
27, 25, and 23 inches. The following table Fig. 9. Atmometer.
indicates the decrease in pressure which
is due to altitude.
ALTITUDE PRESSURE
Feet Meters Inches Centimeters
0 0 30 76
910 277 29 73.5
1850 574 28 71
2820 860 27 68.5
3820 1165 26 66
4850 1477 25 63.5
5910 1792 24 61
7010 2138 23 58.5
8150 2485 22 56
9330 2845 21 53.5
10550 3217 20 51
13170 4016 18 46
16000 4880 16 41
The fluctuations of pressure due to weather are usually so slight that their influence
may be disregarded. An excellent series of tables of relative humidity is found in
Marvin’s Psychrometric Tables, published by the U. S. Weather Bureau, and to be
obtained from the Division of Publications, Washington, D. C., for 10 cents. A
convenient field form is made by removing the portion containing the tables of relative
humidity, and binding it in stiff oilcloth.
74. Sums, means, and curves. An approximate humidity sum can be obtained by
adding the absolute humidities for each of the twenty-four hours, and expressing the
results in grains per cubic foot. It is possible to establish a general ratio between this sum
and the transpiration sum of the plant, but its value is not great at present. Means of
absolute and of relative humidity are readily determinable from the psychrograph
records; the latter are the most useful. The mean of relative humidity for the twenty-four
hours of a day is the average of the twenty-four hour humidities. From these means the
seasonal mean is computed in the same manner. A close approximation, usually within 1
degree, may be obtained in either case by averaging the maximum and minimum for the
period concerned. Various kinds of curves are of value in representing variation in
humidity. Obviously, these must be derived from the psychrograph, or from the
psychrometer when the series is sufficiently complete. The level curve indicates the
variation in different stations at the same time. These may be combined in a series for the
comparison of readings made at various heights in the stations. The day or point curve
shows the fluctuations during the day of one point, and the station curve the variation at
different heights in the same station. The curves of successive days or of different
stations may of course be combined on the same sheet for comparison. Level and station
curves based upon mean relative humidities are especially valuable.
indicates the decrease in pressure which
is due to altitude.
ALTITUDE PRESSURE
Feet Meters Inches Centimeters
0 0 30 76
910 277 29 73.5
1850 574 28 71
2820 860 27 68.5
3820 1165 26 66
4850 1477 25 63.5
5910 1792 24 61
7010 2138 23 58.5
8150 2485 22 56
9330 2845 21 53.5
10550 3217 20 51
13170 4016 18 46
16000 4880 16 41
The fluctuations of pressure due to weather are usually so slight that their influence
may be disregarded. An excellent series of tables of relative humidity is found in
Marvin’s Psychrometric Tables, published by the U. S. Weather Bureau, and to be
obtained from the Division of Publications, Washington, D. C., for 10 cents. A
convenient field form is made by removing the portion containing the tables of relative
humidity, and binding it in stiff oilcloth.
74. Sums, means, and curves. An approximate humidity sum can be obtained by
adding the absolute humidities for each of the twenty-four hours, and expressing the
results in grains per cubic foot. It is possible to establish a general ratio between this sum
and the transpiration sum of the plant, but its value is not great at present. Means of
absolute and of relative humidity are readily determinable from the psychrograph
records; the latter are the most useful. The mean of relative humidity for the twenty-four
hours of a day is the average of the twenty-four hour humidities. From these means the
seasonal mean is computed in the same manner. A close approximation, usually within 1
degree, may be obtained in either case by averaging the maximum and minimum for the
period concerned. Various kinds of curves are of value in representing variation in
humidity. Obviously, these must be derived from the psychrograph, or from the
psychrometer when the series is sufficiently complete. The level curve indicates the
variation in different stations at the same time. These may be combined in a series for the
comparison of readings made at various heights in the stations. The day or point curve
shows the fluctuations during the day of one point, and the station curve the variation at
different heights in the same station. The curves of successive days or of different
stations may of course be combined on the same sheet for comparison. Level and station
curves based upon mean relative humidities are especially valuable.
Page 75
75. Records. A field form is obviously
unnecessary for the psychrograph. The
record sheets constitute both a field and
permanent record. The altitude and other
constant features of the station and the list
of species, etc., are entered on the back of
the first record sheet, or, better, they are
noted in the permanent formation record.
For psychrometer readings, whether
single or in series, the following record
form is employed:
Fig. 10. Conversion scale for temperatures.
unnecessary for the psychrograph. The
record sheets constitute both a field and
permanent record. The altitude and other
constant features of the station and the list
of species, etc., are entered on the back of
the first record sheet, or, better, they are
noted in the permanent formation record.
For psychrometer readings, whether
single or in series, the following record
form is employed:
Fig. 10. Conversion scale for temperatures.
Page 76
Height of Dry Wet Rel. Base Abs. NOTES
Date Hour Formation Station Altitude Community Diff.
reading bulb bulb Hum. Hum. Hum. Sky Rain Wind
6:20 Brook 2500
15/8/’04 Spruce Mertensiare 1 ft. 51° 46° 5 72% 63% 2.9 Clear 0 0
A.M. bank m
Half
„ „ Hiawatha „ Asterare „ 56° 49° 7 64% 63% 3.0 „ 0 0
gravel
6:45 Brook 2
„ Spruce „ Mertensiare „ 54° 52° 2 89% 69% 4.2 „ 0
P.M. bank cc.
Half 2
„ „ Hiawatha „ Asterare „ 56° 52° 4 79% 69% 4.0 „ 0
gravel cc.
On page 47 is given a table for the conversion of Centigrade into Fahrenheit
temperatures. This may be done mentally by means of the formula F = C/5 × 9 + 32°.
Date Hour Formation Station Altitude Community Diff.
reading bulb bulb Hum. Hum. Hum. Sky Rain Wind
6:20 Brook 2500
15/8/’04 Spruce Mertensiare 1 ft. 51° 46° 5 72% 63% 2.9 Clear 0 0
A.M. bank m
Half
„ „ Hiawatha „ Asterare „ 56° 49° 7 64% 63% 3.0 „ 0 0
gravel
6:45 Brook 2
„ Spruce „ Mertensiare „ 54° 52° 2 89% 69% 4.2 „ 0
P.M. bank cc.
Half 2
„ „ Hiawatha „ Asterare „ 56° 52° 4 79% 69% 4.0 „ 0
gravel cc.
On page 47 is given a table for the conversion of Centigrade into Fahrenheit
temperatures. This may be done mentally by means of the formula F = C/5 × 9 + 32°.
Page 77
LIGHT
76. Methods. All methods for measuring light intensity, which have been at all
satisfactory, are based upon the fact that silver salts blacken in the light. The first
photographic method was proposed by Bunsen and Roscoe in 1862; this has been taken
up by Wiesner and variously modified. After considerable experiment by the writer,
however, it seemed desirable to abandon all methods which require the use of “normal
paper” and “normal black” and to develop a simpler one. As space is lacking for a
satisfactory discussion of the Bunsen-Roscoe-Wiesner methods, the reader is referred to
the works cited below.[4] Simple photometers for making light readings simultaneously or
in series were constructed in 1900, and have been in constant use since that time. An
automatic instrument capable of making accurate continuous records proved to be a more
difficult problem. A sunshine recorder was ultimately found which yields valuable
results, and very recently a recording photometer which promises to be perfectly
satisfactory has been devised. Since the hourly and daily variations of sunlight in the
same habitat are relatively small, automatic photometers are perhaps a convenience
rather than a necessity.
The Photometer
76. Methods. All methods for measuring light intensity, which have been at all
satisfactory, are based upon the fact that silver salts blacken in the light. The first
photographic method was proposed by Bunsen and Roscoe in 1862; this has been taken
up by Wiesner and variously modified. After considerable experiment by the writer,
however, it seemed desirable to abandon all methods which require the use of “normal
paper” and “normal black” and to develop a simpler one. As space is lacking for a
satisfactory discussion of the Bunsen-Roscoe-Wiesner methods, the reader is referred to
the works cited below.[4] Simple photometers for making light readings simultaneously or
in series were constructed in 1900, and have been in constant use since that time. An
automatic instrument capable of making accurate continuous records proved to be a more
difficult problem. A sunshine recorder was ultimately found which yields valuable
results, and very recently a recording photometer which promises to be perfectly
satisfactory has been devised. Since the hourly and daily variations of sunlight in the
same habitat are relatively small, automatic photometers are perhaps a convenience
rather than a necessity.
The Photometer
Page 78
Fig. 11. Photometer, showing front and side view.
77. Construction. The simple form of photometer shown in the illustration is a light-
tight metal box with a central wheel upon which a strip of photographic paper is
fastened. This wheel is revolved by the thumbscrew past an opening 6 mm. square which
is closed by means of a slide working closely between two flanges. At the edge of the
opening, and beneath the slide is a hollow for the reception of a permanent light
standard. The disk of the thumbscrew is graduated into twenty-five parts, and these are
numbered. A line just beneath the opening coincides with the successive lines on the
disk, and indicates the number of the exposure. The wheel contains twenty-five hollows
in which the click works, thus moving each exposure just beyond the opening. The metal
case is made in two parts, so that the bottom may be readily removed, and the
photographic strip placed in position. The water-photometer is similar except that the
opening is always covered with a transparent strip and the whole instrument is water-
tight. These instruments have been made especially for measuring light by the C. H.
Stoelting Co., 31 W. Randolph street, Chicago, Ill. The price is $5.
78. Filling the photometer. The photographic paper called “solio” which is made by
the Eastman Kodak Company, Rochester, N. Y., has proved to be much the best for
photometric readings. The most convenient size is that of the 8 × 10 inch sheet, which
77. Construction. The simple form of photometer shown in the illustration is a light-
tight metal box with a central wheel upon which a strip of photographic paper is
fastened. This wheel is revolved by the thumbscrew past an opening 6 mm. square which
is closed by means of a slide working closely between two flanges. At the edge of the
opening, and beneath the slide is a hollow for the reception of a permanent light
standard. The disk of the thumbscrew is graduated into twenty-five parts, and these are
numbered. A line just beneath the opening coincides with the successive lines on the
disk, and indicates the number of the exposure. The wheel contains twenty-five hollows
in which the click works, thus moving each exposure just beyond the opening. The metal
case is made in two parts, so that the bottom may be readily removed, and the
photographic strip placed in position. The water-photometer is similar except that the
opening is always covered with a transparent strip and the whole instrument is water-
tight. These instruments have been made especially for measuring light by the C. H.
Stoelting Co., 31 W. Randolph street, Chicago, Ill. The price is $5.
78. Filling the photometer. The photographic paper called “solio” which is made by
the Eastman Kodak Company, Rochester, N. Y., has proved to be much the best for
photometric readings. The most convenient size is that of the 8 × 10 inch sheet, which
Page 79
can be obtained at any supply house in packages of a dozen sheets for 60 cents. New
“emulsions,” i. e., new lots of paper, are received by the dealers every week, but each
emulsion can be preserved for three to six months without harm if kept in a cool, light-
tight place. Furthermore, all emulsions are made in exactly the same way, and it has been
impossible to detect any difference in them. To fill the photometer, a strip exactly 6 mm.
wide is cut lengthwise from the 8 × 10 sheet. This must be done in the dark room, or at
night in very weak light. The strip is placed on the wheel, extreme care being taken not to
touch the coated surface, and fixed in position by forcing the free ends into the slit of the
wheel by a piece of cork 8–9 mm. long. The wheel is replaced in the case, turned until
the zero is opposite the index line, and the instrument is ready for use.
79. Making readings. An exposure is made by moving the slide quickly in such a way
as to uncover the entire opening, and the standard if the exposure is to be very short.
Care must be taken not to pull the slide entirely out of the groove, as it will be impossible
to replace it with sufficient quickness. The time of exposure can be determined by any
watch after a little practice. It is somewhat awkward for one person to manage the slide
properly when his attention is fixed upon a second hand. This is obviated by having one
observer handle the watch and another the photometer, but here the reaction time is a
source of considerable error. The most satisfactory method is to use a stop-watch. This
can be held in the left hand and started and stopped by the index finger. The photometer
is held against it in the right hand in such a way that the two movements of stopping the
watch and closing the slide may be made at the same instant. The length of exposure is
that necessary to bring the tint of the paper to that of the standard beside it. A second
method which is equally advantageous and sometimes preferable does away with the
permanent standard in the field and the need for a stop-watch. In this event, the strip is
exposed until a medium color is obtained, since very light or very deep prints are harder
to match. This is later compared with the multiple standard. In both cases, the date, time
of day, station, number of instrument and of exposure, and the length of the latter in
seconds are carefully noted. The instrument is held with the edge toward the south at the
level to be read, and the opening uppermost in the usual position of the leaf. When
special readings are desired, as for isophotic leaves, reflected light, etc., the position is
naturally changed to correspond. In practice, it is made an invariable rule to move the
strip for the next exposure as soon as the slide is closed. Otherwise double exposures are
liable to occur. When a strip is completely exposed it is removed in the dark, and a new
one put in place. The former is carefully labeled and dated on the back, and put away in a
light-tight box in a cool place.
“emulsions,” i. e., new lots of paper, are received by the dealers every week, but each
emulsion can be preserved for three to six months without harm if kept in a cool, light-
tight place. Furthermore, all emulsions are made in exactly the same way, and it has been
impossible to detect any difference in them. To fill the photometer, a strip exactly 6 mm.
wide is cut lengthwise from the 8 × 10 sheet. This must be done in the dark room, or at
night in very weak light. The strip is placed on the wheel, extreme care being taken not to
touch the coated surface, and fixed in position by forcing the free ends into the slit of the
wheel by a piece of cork 8–9 mm. long. The wheel is replaced in the case, turned until
the zero is opposite the index line, and the instrument is ready for use.
79. Making readings. An exposure is made by moving the slide quickly in such a way
as to uncover the entire opening, and the standard if the exposure is to be very short.
Care must be taken not to pull the slide entirely out of the groove, as it will be impossible
to replace it with sufficient quickness. The time of exposure can be determined by any
watch after a little practice. It is somewhat awkward for one person to manage the slide
properly when his attention is fixed upon a second hand. This is obviated by having one
observer handle the watch and another the photometer, but here the reaction time is a
source of considerable error. The most satisfactory method is to use a stop-watch. This
can be held in the left hand and started and stopped by the index finger. The photometer
is held against it in the right hand in such a way that the two movements of stopping the
watch and closing the slide may be made at the same instant. The length of exposure is
that necessary to bring the tint of the paper to that of the standard beside it. A second
method which is equally advantageous and sometimes preferable does away with the
permanent standard in the field and the need for a stop-watch. In this event, the strip is
exposed until a medium color is obtained, since very light or very deep prints are harder
to match. This is later compared with the multiple standard. In both cases, the date, time
of day, station, number of instrument and of exposure, and the length of the latter in
seconds are carefully noted. The instrument is held with the edge toward the south at the
level to be read, and the opening uppermost in the usual position of the leaf. When
special readings are desired, as for isophotic leaves, reflected light, etc., the position is
naturally changed to correspond. In practice, it is made an invariable rule to move the
strip for the next exposure as soon as the slide is closed. Otherwise double exposures are
liable to occur. When a strip is completely exposed it is removed in the dark, and a new
one put in place. The former is carefully labeled and dated on the back, and put away in a
light-tight box in a cool place.
Page 80
Fig. 12. Dawson-Lander sun recorder.
80. The Dawson-Lander sun recorder. “The instrument consists of a small outer
cylinder of copper which revolves with the sun, and through the side of which is cut a
narrow slit to allow the sunshine to impinge on a strip of sensitive paper, wound round a
drum which fits closely inside the outer cylinder, but is held by a pin so that it can not
rotate. By means of a screw fixed to the lid of the outer cylinder, the drum holding the
sensitive paper is made to travel endwise down the outer tube, one-eighth of an inch
daily, so that a fresh portion of the sensitive surface is brought into position to receive the
record.” The instrument is driven by an eight-day clock placed in the base below the
drum. The slit is covered by means of a flattened funnel-shaped hood, and the
photographic strip is protected from rain by a perfectly transparent sheet of celluloid. The
detailed structure of the instrument is shown in figure 12. This instrument may be
obtained from Lander and Smith, Canterbury, England, for $35.
In setting up the sunshine recorder, the axis should be placed in such a position that the
angle which it makes with the base is the same as the altitude of the place where the
observations are made. This is readily done by loosening the bolts at either side. The
drum is removed, the celluloid sheet unwound by means of the key which holds it in
place, the sensitive strip put in position, and the sheet again wound up. Strips of a special
sensitive paper upon which the hours are indicated are furnished by the makers of the
80. The Dawson-Lander sun recorder. “The instrument consists of a small outer
cylinder of copper which revolves with the sun, and through the side of which is cut a
narrow slit to allow the sunshine to impinge on a strip of sensitive paper, wound round a
drum which fits closely inside the outer cylinder, but is held by a pin so that it can not
rotate. By means of a screw fixed to the lid of the outer cylinder, the drum holding the
sensitive paper is made to travel endwise down the outer tube, one-eighth of an inch
daily, so that a fresh portion of the sensitive surface is brought into position to receive the
record.” The instrument is driven by an eight-day clock placed in the base below the
drum. The slit is covered by means of a flattened funnel-shaped hood, and the
photographic strip is protected from rain by a perfectly transparent sheet of celluloid. The
detailed structure of the instrument is shown in figure 12. This instrument may be
obtained from Lander and Smith, Canterbury, England, for $35.
In setting up the sunshine recorder, the axis should be placed in such a position that the
angle which it makes with the base is the same as the altitude of the place where the
observations are made. This is readily done by loosening the bolts at either side. The
drum is removed, the celluloid sheet unwound by means of the key which holds it in
place, the sensitive strip put in position, and the sheet again wound up. Strips of a special
sensitive paper upon which the hours are indicated are furnished by the makers of the
Page 81
instrument, but it has been found preferable to use solio strips in order to facilitate
comparison with the standards. The drum is placed on the axis, and is screwed up until it
just escapes the collar at the top of the spiral. The clock is wound and started, and the
outer cylinder put on so that the proper hour mark coincides with the index on the front
of the base.
As a sunshine recorder, the instrument gives a perfect record, in which the varying
intensities are readily recognizable. Since the cylinder moves one-half inch in an hour,
and the slit is .01 of an inch, the time of each exposure is 72 seconds. This gives a very
deep color on the solio paper, which results in a serious error in making comparisons
with the standard. On account of the hood, diffuse light is not recorded when it is too
weak to cast a distinct shadow. It seems probable that this difficulty will be overcome by
the use of a flat disk containing the proper slit, and in this event the instrument will
become of especial value for measuring the diffuse light of layered formations. The
celluloid sheet constitutes a source of error in sunlight on account of the reflection which
it causes. This can be prevented by using the instrument only on sunny days, when the
protection of the sheet can be dispensed with.
81. The selagraph. This instrument is at present under construction, and can only be
described in a general way. In principle it is a simple photometer operating automatically.
It consists of a light-tight box preferably of metal, which contains an eight-day lever
clock. Attached to the arbor of the latter is a disk 7 inches in diameter bearing on its
circumference a solio strip 1 cm. wide and 59 cm. long. The opening in the box for
exposure is 6 mm. square and is controlled by a photographic shutter. The latter is
constructed so that it may be set for 5, 10, or 20 seconds, since a single period of
exposure can not serve for both sun and shade. The shutter is tripped once every two
hours, by means of a special wheel revolving once a day. Each exposure is 6 mm. square,
and is separated by a small space from the next one. Twelve exposures are made every 24
hours, and 84 during the week, though, naturally, the daytime exposures alone are
recorded. Comparisons with the multiple standard are made exactly as in the case of the
simple photometer. The selagraph is made by the C. H. Stoelting Co., Chicago, Illinois.
Standards
82. Use. The light value of each exposure is determined by reference to a standard.
When the photometer carries a permanent standard, each exposure is brought to the tint
of the latter, and its value is indicated by the time ratio between them. Thus, if the
standard is the result of a 5–second exposure to full sunlight at meridian, and a reading
which corresponds in color requires 100 seconds in the habitat concerned, the light of the
latter is twenty times weaker or more diffuse. Usually, the standard is regarded as unity,
and light values figured with reference to it, as .05. With the selagraph such a use of the
standard is impossible, and often, also, with the photometer it is unnecessary or not
desirable. The value of each exposure in such case is obtained by matching it with a
multiple standard, after the entire strip has been exposed. The further steps are those
comparison with the standards. The drum is placed on the axis, and is screwed up until it
just escapes the collar at the top of the spiral. The clock is wound and started, and the
outer cylinder put on so that the proper hour mark coincides with the index on the front
of the base.
As a sunshine recorder, the instrument gives a perfect record, in which the varying
intensities are readily recognizable. Since the cylinder moves one-half inch in an hour,
and the slit is .01 of an inch, the time of each exposure is 72 seconds. This gives a very
deep color on the solio paper, which results in a serious error in making comparisons
with the standard. On account of the hood, diffuse light is not recorded when it is too
weak to cast a distinct shadow. It seems probable that this difficulty will be overcome by
the use of a flat disk containing the proper slit, and in this event the instrument will
become of especial value for measuring the diffuse light of layered formations. The
celluloid sheet constitutes a source of error in sunlight on account of the reflection which
it causes. This can be prevented by using the instrument only on sunny days, when the
protection of the sheet can be dispensed with.
81. The selagraph. This instrument is at present under construction, and can only be
described in a general way. In principle it is a simple photometer operating automatically.
It consists of a light-tight box preferably of metal, which contains an eight-day lever
clock. Attached to the arbor of the latter is a disk 7 inches in diameter bearing on its
circumference a solio strip 1 cm. wide and 59 cm. long. The opening in the box for
exposure is 6 mm. square and is controlled by a photographic shutter. The latter is
constructed so that it may be set for 5, 10, or 20 seconds, since a single period of
exposure can not serve for both sun and shade. The shutter is tripped once every two
hours, by means of a special wheel revolving once a day. Each exposure is 6 mm. square,
and is separated by a small space from the next one. Twelve exposures are made every 24
hours, and 84 during the week, though, naturally, the daytime exposures alone are
recorded. Comparisons with the multiple standard are made exactly as in the case of the
simple photometer. The selagraph is made by the C. H. Stoelting Co., Chicago, Illinois.
Standards
82. Use. The light value of each exposure is determined by reference to a standard.
When the photometer carries a permanent standard, each exposure is brought to the tint
of the latter, and its value is indicated by the time ratio between them. Thus, if the
standard is the result of a 5–second exposure to full sunlight at meridian, and a reading
which corresponds in color requires 100 seconds in the habitat concerned, the light of the
latter is twenty times weaker or more diffuse. Usually, the standard is regarded as unity,
and light values figured with reference to it, as .05. With the selagraph such a use of the
standard is impossible, and often, also, with the photometer it is unnecessary or not
desirable. The value of each exposure in such case is obtained by matching it with a
multiple standard, after the entire strip has been exposed. The further steps are those
Page 82
already indicated. After the exact tint in the standard has been found, the length of the
reading in seconds is divided by the time of the proper standard, and the result expressed
as above.
83. Making a standard. Standards are obtained by exposing the photometer at
meridian on a typically clear day, and in the field where there is the least dust and smoke.
Exception to the latter may be made, of course, in obtaining standards for plant houses
located in cities, though it is far better to have the same one for both field and control
experiment. Usable standards can be obtained on any bright day at the base station.
Indeed, valuable results are often secured by immediate successive sun and shade
readings in adjacent habitats, where the sun reading series is the sole standard.
Preferably, standards should be made at the solstices or equinoxes, and at a
representative station. The June solstice is much to be preferred, as it represents the
maximum light values of the year. Lincoln has been taken as the base station for the
plains and mountains. It is desirable, however, that a national or international station be
ultimately selected for this purpose, in order that light values taken in different parts of
the world may be readily compared.
84. Kinds of standards. The base standard is the one taken at Lincoln (latitude 41°
N.) at meridian June 20–22. This is properly the unit to which all exposures are referred,
but it has been found convenient to employ the Minnehaha standard as the base for the
Colorado mountains, in order to avoid reducing each time. Relative standards are
frequently used for temporary purposes. Thus, in comparing the light intensities of a
series of formations, one to five standards are exposed on the solio strip before beginning
the series of readings. Proof standards are the exposed solio strips, which fade in the
light, and can, in consequence, be kept only a few weeks without possibility of error. The
fading can be prevented by “toning” the strip, but in this event the exposures must be
fixed in like manner before they can be compared. This process is inconvenient and time-
consuming. It is also open to considerable error, as the time of treatment, strength of
solution, etc., must be exactly equivalent in all instances. Permanent standards are
accurate water-color copies of the originals obtained by the photometer. These have the
apparent disadvantage of requiring a double comparison or matching, but after a little
practice it is possible to reproduce the solio tints so that the copy is practically
indistinguishable from the original. The most satisfactory method is to make a long
stroke of color on a pure white paper, since a broad wash is not quite homogeneous, and
then to reject such parts of the stroke as do not match exactly. Permanent standards fade
after a few month’s use, and must be replaced by parts of the original stroke. Single
standards are made by one exposure, while multiple ones have a series of exposures
filling a whole light strip. These are regularly obtained by making the exposures from 1–
10 seconds respectively, and then increasing the length of each successive exposure by 2
seconds. Single exposures of 1–5 seconds as desired usually serve as the basis for
permanent standards, but a multiple standard may also be copied in permanent form.
Exposures for securing standards must be made only under the most favorable
conditions, and the length in seconds must be exact. The use of the stop-watch is
imperative, except where access may be had to an astronomical clock with a large second
reading in seconds is divided by the time of the proper standard, and the result expressed
as above.
83. Making a standard. Standards are obtained by exposing the photometer at
meridian on a typically clear day, and in the field where there is the least dust and smoke.
Exception to the latter may be made, of course, in obtaining standards for plant houses
located in cities, though it is far better to have the same one for both field and control
experiment. Usable standards can be obtained on any bright day at the base station.
Indeed, valuable results are often secured by immediate successive sun and shade
readings in adjacent habitats, where the sun reading series is the sole standard.
Preferably, standards should be made at the solstices or equinoxes, and at a
representative station. The June solstice is much to be preferred, as it represents the
maximum light values of the year. Lincoln has been taken as the base station for the
plains and mountains. It is desirable, however, that a national or international station be
ultimately selected for this purpose, in order that light values taken in different parts of
the world may be readily compared.
84. Kinds of standards. The base standard is the one taken at Lincoln (latitude 41°
N.) at meridian June 20–22. This is properly the unit to which all exposures are referred,
but it has been found convenient to employ the Minnehaha standard as the base for the
Colorado mountains, in order to avoid reducing each time. Relative standards are
frequently used for temporary purposes. Thus, in comparing the light intensities of a
series of formations, one to five standards are exposed on the solio strip before beginning
the series of readings. Proof standards are the exposed solio strips, which fade in the
light, and can, in consequence, be kept only a few weeks without possibility of error. The
fading can be prevented by “toning” the strip, but in this event the exposures must be
fixed in like manner before they can be compared. This process is inconvenient and time-
consuming. It is also open to considerable error, as the time of treatment, strength of
solution, etc., must be exactly equivalent in all instances. Permanent standards are
accurate water-color copies of the originals obtained by the photometer. These have the
apparent disadvantage of requiring a double comparison or matching, but after a little
practice it is possible to reproduce the solio tints so that the copy is practically
indistinguishable from the original. The most satisfactory method is to make a long
stroke of color on a pure white paper, since a broad wash is not quite homogeneous, and
then to reject such parts of the stroke as do not match exactly. Permanent standards fade
after a few month’s use, and must be replaced by parts of the original stroke. Single
standards are made by one exposure, while multiple ones have a series of exposures
filling a whole light strip. These are regularly obtained by making the exposures from 1–
10 seconds respectively, and then increasing the length of each successive exposure by 2
seconds. Single exposures of 1–5 seconds as desired usually serve as the basis for
permanent standards, but a multiple standard may also be copied in permanent form.
Exposures for securing standards must be made only under the most favorable
conditions, and the length in seconds must be exact. The use of the stop-watch is
imperative, except where access may be had to an astronomical clock with a large second
Page 83
hand, which is even more satisfactory. The length of time necessary for the series desired
is reckoned beforehand, and the exposures begun so that the meridian falls in the middle
of the process.
Single standards are exceedingly convenient in photometer readings, but they are open
to one objection. In the sunshine it is necessary to make instant decision upon the
accuracy of the match, or the exposure becomes too deep. In the shade where the action
is slower, this difficulty is not felt. For this reason it is usually desirable to check the
results by a multiple standard, and in the case of selagraph records, where the various
exposures show a wide range of tint, light values are obtainable only by direct
comparison with the multiple standard. The exact matching of exposure and standard
requires great accuracy, but with a little practice this may be done with slight chance of
error by merely moving the exposure along the various tints of the standard until the
proper shade is found. The requisite skill is soon acquired by running over a strip of
exposures several times until the comparisons always yield the same results for each. The
margin of error is practically negligible when the same person makes all the
comparisons, and in the case of two or three working on the same reading the results
diverge little or not at all. The efficient difference for light is much more of a variable
than is the case with water-content. It has been determined so far only for a few species,
all of which seem to indicate that appreciable modification in the form or structure of a
leaf does not occur until the reduction in intensity reaches .1 of the meridian sunlight at
the June solstice. The error of comparison is far less than this, and consequently may be
ignored, even in the most painstaking inquiry.
Readings
85. Time. The intensity of the light incident upon a habitat varies periodically with the
hour and the day, and changes in accord with the changing conditions of the sky. The
light variations on cloudy days can only be determined by the photometer. While these
can not be ignored, proper comparisons can be instituted only between the readings taken
on normal days of sunshine. The sunlight varies with the altitude of the sun, i. e., the
angle which its rays make with the surface at a given latitude. This angle reaches a daily
maximum at meridian. The yearly maximum falls on June 22, and the angle decreases in
both directions through the year to a minimum on December 22. At equal distances from
either solstice, the angle is the same, e. g., on March 21 and September 23. At Lincoln
(41° N. latitude) the extremes at meridian are 73° and 26°; at Minnehaha (39°) they are
75° and 28°. The extremes for any latitude may be found by subtracting its distance in
degrees north of the two tropics from 90. Thus, the 50th parallel is 26.5° north of the
tropic of Cancer, and the maximum altitude of the sun at a place upon it is 63.5°. It is
73.5° north of the tropic of Capricorn, and the minimum meridional altitude is 16.5°.
The changes in the amount of light due to the altitude of the sun are produced by the
earth’s atmosphere. The absorption of light rays is greatest near the horizon, where their
pathway through the atmosphere is longest, and it is least at the zenith. The absorption,
is reckoned beforehand, and the exposures begun so that the meridian falls in the middle
of the process.
Single standards are exceedingly convenient in photometer readings, but they are open
to one objection. In the sunshine it is necessary to make instant decision upon the
accuracy of the match, or the exposure becomes too deep. In the shade where the action
is slower, this difficulty is not felt. For this reason it is usually desirable to check the
results by a multiple standard, and in the case of selagraph records, where the various
exposures show a wide range of tint, light values are obtainable only by direct
comparison with the multiple standard. The exact matching of exposure and standard
requires great accuracy, but with a little practice this may be done with slight chance of
error by merely moving the exposure along the various tints of the standard until the
proper shade is found. The requisite skill is soon acquired by running over a strip of
exposures several times until the comparisons always yield the same results for each. The
margin of error is practically negligible when the same person makes all the
comparisons, and in the case of two or three working on the same reading the results
diverge little or not at all. The efficient difference for light is much more of a variable
than is the case with water-content. It has been determined so far only for a few species,
all of which seem to indicate that appreciable modification in the form or structure of a
leaf does not occur until the reduction in intensity reaches .1 of the meridian sunlight at
the June solstice. The error of comparison is far less than this, and consequently may be
ignored, even in the most painstaking inquiry.
Readings
85. Time. The intensity of the light incident upon a habitat varies periodically with the
hour and the day, and changes in accord with the changing conditions of the sky. The
light variations on cloudy days can only be determined by the photometer. While these
can not be ignored, proper comparisons can be instituted only between the readings taken
on normal days of sunshine. The sunlight varies with the altitude of the sun, i. e., the
angle which its rays make with the surface at a given latitude. This angle reaches a daily
maximum at meridian. The yearly maximum falls on June 22, and the angle decreases in
both directions through the year to a minimum on December 22. At equal distances from
either solstice, the angle is the same, e. g., on March 21 and September 23. At Lincoln
(41° N. latitude) the extremes at meridian are 73° and 26°; at Minnehaha (39°) they are
75° and 28°. The extremes for any latitude may be found by subtracting its distance in
degrees north of the two tropics from 90. Thus, the 50th parallel is 26.5° north of the
tropic of Cancer, and the maximum altitude of the sun at a place upon it is 63.5°. It is
73.5° north of the tropic of Capricorn, and the minimum meridional altitude is 16.5°.
The changes in the amount of light due to the altitude of the sun are produced by the
earth’s atmosphere. The absorption of light rays is greatest near the horizon, where their
pathway through the atmosphere is longest, and it is least at the zenith. The absorption,
Page 84
and, consequently, the relative intensity of sunlight, can be determined at a given place
for each hour of any sunshiny day by the use of chart 13. This chart has been constructed
for Lincoln, and will serve for all places within a few degrees of the 40th parallel. The
curves which show the altitude of the sun at the various times of the day and the year
have been constructed by measurements upon the celestial globe. Each interval between
the horizontal lines represents 2 degrees of the sun’s altitude. The vertical lines indicate
time before or after the apparent noon, the intervals corresponding to 10 minutes. If the
relative intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent noon for
this day must first be determined. A glance at the table shows that the sun crosses the
meridian on this day at 9 minutes 53 seconds past noon at the 90th meridian. The
apparent noon at Lincoln is found by adding 26 minutes 49 seconds, the difference in
time between Lincoln and a point on the 90th meridian. When the sun is fast, the proper
number of minutes is taken from 26 minutes 49 seconds. The apparent noon on March 12
is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23 minutes later. The sun’s
altitude is accordingly 36°. If the intensity of the light which reaches the earth’s surface
when the sun is at zenith is taken as 1, the table of the sun’s altitudes gives the intensity
at 3:00 P.M. on March 12 as .85.
For places with a latitude differing by several degrees from that of Lincoln, it is
necessary to construct a new table of altitude curves from the celestial globe. It is quite
possible to make a close approximation of this from the table given, since the maximum
and minimum meridional altitude, and hence the corresponding light intensity, can be
obtained as indicated above. For Minnehaha, which is on the 105th meridian, and for
other places on standard meridians, i. e., 60°, 75°, 90°, and 120° W., the table of apparent
noon indicates the number of minutes to be added to 12 noon, standard time, when the
sun is slow, and to be subtracted when the sun is fast. The time at a place east or west of
a standard meridian is respectively faster or slower than the latter. The exact difference in
minutes is obtained from the difference in longitude by the equation, 15° = 1 hour. Thus,
Lincoln, 96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is consequently 26
minutes 49 seconds slower, and this time must always be added to the apparent noon as
determined from the chart. At a place east of a standard meridian, the time difference is,
of course, subtracted.
for each hour of any sunshiny day by the use of chart 13. This chart has been constructed
for Lincoln, and will serve for all places within a few degrees of the 40th parallel. The
curves which show the altitude of the sun at the various times of the day and the year
have been constructed by measurements upon the celestial globe. Each interval between
the horizontal lines represents 2 degrees of the sun’s altitude. The vertical lines indicate
time before or after the apparent noon, the intervals corresponding to 10 minutes. If the
relative intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent noon for
this day must first be determined. A glance at the table shows that the sun crosses the
meridian on this day at 9 minutes 53 seconds past noon at the 90th meridian. The
apparent noon at Lincoln is found by adding 26 minutes 49 seconds, the difference in
time between Lincoln and a point on the 90th meridian. When the sun is fast, the proper
number of minutes is taken from 26 minutes 49 seconds. The apparent noon on March 12
is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23 minutes later. The sun’s
altitude is accordingly 36°. If the intensity of the light which reaches the earth’s surface
when the sun is at zenith is taken as 1, the table of the sun’s altitudes gives the intensity
at 3:00 P.M. on March 12 as .85.
For places with a latitude differing by several degrees from that of Lincoln, it is
necessary to construct a new table of altitude curves from the celestial globe. It is quite
possible to make a close approximation of this from the table given, since the maximum
and minimum meridional altitude, and hence the corresponding light intensity, can be
obtained as indicated above. For Minnehaha, which is on the 105th meridian, and for
other places on standard meridians, i. e., 60°, 75°, 90°, and 120° W., the table of apparent
noon indicates the number of minutes to be added to 12 noon, standard time, when the
sun is slow, and to be subtracted when the sun is fast. The time at a place east or west of
a standard meridian is respectively faster or slower than the latter. The exact difference in
minutes is obtained from the difference in longitude by the equation, 15° = 1 hour. Thus,
Lincoln, 96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is consequently 26
minutes 49 seconds slower, and this time must always be added to the apparent noon as
determined from the chart. At a place east of a standard meridian, the time difference is,
of course, subtracted.
Page 85
Fig. 13. Chart for the determination of the sun’s altitude, and the corresponding light intensity.
The actual differences in the light intensity from hour to hour and day to day, which
are caused by variations in the sun’s altitude, are not as great as might be expected. For
example, the maximum intensity at Lincoln, June 22, is .98; the minimum meridional
intensity December 22 is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00
A.M. and 6:00 P.M. approximately); between 8:00 A.M. and 4:00 P.M. the range in intensity
is from .90 to .98 merely. On December 22, the greatest intensity is .52, the least .20 (the
latter at 8:00 A.M. and 4:00 P.M. approximately). If the growing season be taken as
beginning with the 1st of March and closing the 1st of October, the greatest variation in
light intensity at Lincoln within a period of 10 hours with the meridian at its center
(cloudy days excepted) is from .33 to .98. In a period of 8 hours, the extremes are .65 to
.98, i. e., the greatest variation, .3, is far within the efficient difference, which has been
put at .9. For the growing period, then, readings made between 8:00 A.M. and 4:00 P.M. on
normal sunshiny days may be compared directly, without taking into account the
compensation for the sun’s altitude. Until the efficient difference has been determined for
a large number of species, however, it seems wise to err on the safe side and to
compensate for great differences in time of day or year. In all doubtful cases, the
The actual differences in the light intensity from hour to hour and day to day, which
are caused by variations in the sun’s altitude, are not as great as might be expected. For
example, the maximum intensity at Lincoln, June 22, is .98; the minimum meridional
intensity December 22 is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00
A.M. and 6:00 P.M. approximately); between 8:00 A.M. and 4:00 P.M. the range in intensity
is from .90 to .98 merely. On December 22, the greatest intensity is .52, the least .20 (the
latter at 8:00 A.M. and 4:00 P.M. approximately). If the growing season be taken as
beginning with the 1st of March and closing the 1st of October, the greatest variation in
light intensity at Lincoln within a period of 10 hours with the meridian at its center
(cloudy days excepted) is from .33 to .98. In a period of 8 hours, the extremes are .65 to
.98, i. e., the greatest variation, .3, is far within the efficient difference, which has been
put at .9. For the growing period, then, readings made between 8:00 A.M. and 4:00 P.M. on
normal sunshiny days may be compared directly, without taking into account the
compensation for the sun’s altitude. Until the efficient difference has been determined for
a large number of species, however, it seems wise to err on the safe side and to
compensate for great differences in time of day or year. In all doubtful cases, the
Page 86
intensity obtained by the astronomical method should also be checked by photometric
readings. A slight error probably enters in, due to reflection from the surface of the paper,
and to temperature, but this is negligible.
86. Table for determining apparent noon
readings. A slight error probably enters in, due to reflection from the surface of the paper,
and to temperature, but this is negligible.
86. Table for determining apparent noon
Page 87
DATE TIME EQUATION LINCOLN NOON
Sun slow: + 26m. 49s.
January 1 3m. 47s. 12:31 P.M.
„ 6 6 7 :33
„ 11 8 12 :35
„ 16 10 3 :37
„ 21 11 35 :38
„ 26 12 48 :40
„ 31 13 41 :40
February 10 14 27 :41
„ 20 13 56 :41
March 2 12 18 :39
„ 7 11 10 :38
„ 12 9 53 :37
„ 17 8 29 :36
„ 22 6 59 :34
„ 27 5 27 :32
April 1 3 55 :31
„ 6 2 27 :29
„ 11 1 3 :28
Sun fast: −
„ 16 0 13 :27
„ 21 1 20 :25
„ 26 2 16 :24
May 1 3 0 :24
„ 16 3 48 :23
„ 31 2 33 :24
June 5 1 45 :25
„ 10 0 49 :26
Sun slow: +
„ 15 0 13 :27
„ 20 1 18 :28
„ 25 2 22 :29
„ 30 3 22 :30
Sun slow: + 26m. 49s.
July 5 4m. 19s. 12:31 P.M.
„ 10 5 7 :32
„ 20 6 6 :33
August 4 5 53 :33
„ 14 4 30 :31
„ 19 3 28 :30
„ 24 2 13 :29
„ 29 0 48 :28
Sun fast: −
September 3 0 45 :26
„ 8 2 25 :24
„ 13 4 9 :23
„ 18 5 55 :21
„ 23 7 41 :19
„ 28 9 23 :17
October 3 10 59 :16
„ 8 12 26 :14
„ 13 13 43 :13
Sun slow: + 26m. 49s.
January 1 3m. 47s. 12:31 P.M.
„ 6 6 7 :33
„ 11 8 12 :35
„ 16 10 3 :37
„ 21 11 35 :38
„ 26 12 48 :40
„ 31 13 41 :40
February 10 14 27 :41
„ 20 13 56 :41
March 2 12 18 :39
„ 7 11 10 :38
„ 12 9 53 :37
„ 17 8 29 :36
„ 22 6 59 :34
„ 27 5 27 :32
April 1 3 55 :31
„ 6 2 27 :29
„ 11 1 3 :28
Sun fast: −
„ 16 0 13 :27
„ 21 1 20 :25
„ 26 2 16 :24
May 1 3 0 :24
„ 16 3 48 :23
„ 31 2 33 :24
June 5 1 45 :25
„ 10 0 49 :26
Sun slow: +
„ 15 0 13 :27
„ 20 1 18 :28
„ 25 2 22 :29
„ 30 3 22 :30
Sun slow: + 26m. 49s.
July 5 4m. 19s. 12:31 P.M.
„ 10 5 7 :32
„ 20 6 6 :33
August 4 5 53 :33
„ 14 4 30 :31
„ 19 3 28 :30
„ 24 2 13 :29
„ 29 0 48 :28
Sun fast: −
September 3 0 45 :26
„ 8 2 25 :24
„ 13 4 9 :23
„ 18 5 55 :21
„ 23 7 41 :19
„ 28 9 23 :17
October 3 10 59 :16
„ 8 12 26 :14
„ 13 13 43 :13
Page 88
„ 18 14 48 :12
„ 23 15 37 :11
November 2 16 20 :10
„ 12 15 45 :11
„ 17 14 54 :12
„ 22 13 44 :13
„ 27 12 14 :15
December 2 10 25 :16
„ 7 8 21 :18
„ 12 6 5 :21
„ 17 3 41 :23
„ 22 1 12 :26
Sun slow: +
„ 27 1 17 :28
87. Place. The effect of latitude upon the sun’s altitude, and the consequent light
intensity have been discussed in the pages which precede. Latitude has also a profound
influence upon the duration of daylight, but the importance of the latter apart from
intensity is not altogether clear. The variation of intensity due to altitude has been greatly
overestimated; it is practically certain, for example, that the dwarf habit of alpine plants
is not to be ascribed to intense illumination, since the latter increases but slightly with the
altitude. It has been demonstrated astronomically that about 20 per cent of a vertical ray
of sunlight is absorbed by the atmosphere by the time it reaches sea level. At the summit
of Pike’s Peak, which is 14,000 feet (4,267 meters) high, the barometric pressure is 17
inches, and the absorption is approximately 11 per cent. In other words, the light at sea
level is 80 per cent of that which enters the earth’s atmosphere; on the summit of Pike’s
Peak it is 89 per cent. As the effect of the sun’s altitude is the same in both places, the
table of curves on page 57 will apply to both. Taking into account the difference in
absorption, the maximum intensity at sea level and at 14,000 feet on the fortieth parallel
is .98 and 1.09 respectively. The minimum intensities between 8:00 A.M. and 4:00 P.M. of
the growing period are .64 and .71 respectively. The correctness of these figures has been
demonstrated by photometer readings, which have given almost exactly the same results.
Such slight variations are quite insufficient to produce an appreciable adjustment,
particularly in structure. They are far within the efficient difference, and Reinke[5] has
found, moreover, that photosynthetic activity in Elodea is not increased beyond the
normal in sunlight sixty times concentrated. In consequence, it is entirely unnecessary to
take account of different altitudes in obtaining light values.
The slope of a habitat exerts a considerable effect upon the intensity of the incident
light. If the angle between the slope and the sun’s ray be 90°, a square meter of surface
will receive the maximum intensity, 1. At an angle of 10°, the same area receives but .17
of the light. This relation between angle and intensity is shown in the table which
follows. The influence of the light, however, is felt by the leaf, not by the slope. Since
there is no connection between the position of the leaf and the slope of the habitat, the
latter may be ignored. In consequence, it is unnecessary to make allowances for the
direction of a slope, viz., whether north, east, south, or west, in so far as light values are
concerned. The angle which a leaf makes with its stem determines the angle of incidence,
„ 23 15 37 :11
November 2 16 20 :10
„ 12 15 45 :11
„ 17 14 54 :12
„ 22 13 44 :13
„ 27 12 14 :15
December 2 10 25 :16
„ 7 8 21 :18
„ 12 6 5 :21
„ 17 3 41 :23
„ 22 1 12 :26
Sun slow: +
„ 27 1 17 :28
87. Place. The effect of latitude upon the sun’s altitude, and the consequent light
intensity have been discussed in the pages which precede. Latitude has also a profound
influence upon the duration of daylight, but the importance of the latter apart from
intensity is not altogether clear. The variation of intensity due to altitude has been greatly
overestimated; it is practically certain, for example, that the dwarf habit of alpine plants
is not to be ascribed to intense illumination, since the latter increases but slightly with the
altitude. It has been demonstrated astronomically that about 20 per cent of a vertical ray
of sunlight is absorbed by the atmosphere by the time it reaches sea level. At the summit
of Pike’s Peak, which is 14,000 feet (4,267 meters) high, the barometric pressure is 17
inches, and the absorption is approximately 11 per cent. In other words, the light at sea
level is 80 per cent of that which enters the earth’s atmosphere; on the summit of Pike’s
Peak it is 89 per cent. As the effect of the sun’s altitude is the same in both places, the
table of curves on page 57 will apply to both. Taking into account the difference in
absorption, the maximum intensity at sea level and at 14,000 feet on the fortieth parallel
is .98 and 1.09 respectively. The minimum intensities between 8:00 A.M. and 4:00 P.M. of
the growing period are .64 and .71 respectively. The correctness of these figures has been
demonstrated by photometer readings, which have given almost exactly the same results.
Such slight variations are quite insufficient to produce an appreciable adjustment,
particularly in structure. They are far within the efficient difference, and Reinke[5] has
found, moreover, that photosynthetic activity in Elodea is not increased beyond the
normal in sunlight sixty times concentrated. In consequence, it is entirely unnecessary to
take account of different altitudes in obtaining light values.
The slope of a habitat exerts a considerable effect upon the intensity of the incident
light. If the angle between the slope and the sun’s ray be 90°, a square meter of surface
will receive the maximum intensity, 1. At an angle of 10°, the same area receives but .17
of the light. This relation between angle and intensity is shown in the table which
follows. The influence of the light, however, is felt by the leaf, not by the slope. Since
there is no connection between the position of the leaf and the slope of the habitat, the
latter may be ignored. In consequence, it is unnecessary to make allowances for the
direction of a slope, viz., whether north, east, south, or west, in so far as light values are
concerned. The angle which a leaf makes with its stem determines the angle of incidence,
Page 89
and hence the amount of light received by the leaf surface. This is relatively unimportant
for two reasons. This angle changes hourly and daily with the altitude of the sun, and the
intensity constantly swings from one extreme to the other. Moreover, the extremes 1.00
and 0.17, even if constant, are hardly sufficient to produce a measurable result. When the
angle of the leaf approaches 90°, there is the well-known differentiation of leaf surfaces
and of chlorenchym, but this has no relation to the angle of incidence.
Table of Intensity at Various Angles
ANGLE INTENSITY
90 1.00
80 .98
70 .94
60 .87
50 .77
40 .64
30 .50
20 .34
10 .17
In the sunlight, it makes no difference at what height a light reading is taken. In forest
and thicket as well as in some herbaceous formations, the intensity of the light, if there is
any difference, is greatest just beneath the foliage of the facies. In forests especially, the
light is increasingly diffuse toward the ground, particularly where layers intervene. In
woodland formations, moreover, the exact spot in which a reading is made must be
carefully chosen, unless the foliage is so dense that the shade is uniform. A very
satisfactory plan is to take readings in two or more spots where the shade appears to be
typical, and to make a check reading in a “sunfleck,” a spot where sunlight shows
through. In forests and thickets, the sunflecks are fleeting, and the light value is
practically that of the shade. In passing into open woodland and thicket, the sunflecks
increase in size and permanence, until finally they exceed the shade areas in amount and
become typical of the formation.
Reflected and Absorbed Light
88. The fate of incident light. The light present in a habitat and incident upon a leaf is
not all available for photosynthesis. Part is reflected or screened out by the epidermis,
and a certain amount passes through the chlorenchym, except in very thick leaves. The
light absorbed is by far the greatest in the majority of species. Many plants with dense
coatings of hairs reflect or withhold more light than they absorb, and the amount of light
reflected by a thick cuticule is likewise great. As light is imponderable, the actual amount
absorbed or reflected by the leaf can not be determined. It is possible, however, to
for two reasons. This angle changes hourly and daily with the altitude of the sun, and the
intensity constantly swings from one extreme to the other. Moreover, the extremes 1.00
and 0.17, even if constant, are hardly sufficient to produce a measurable result. When the
angle of the leaf approaches 90°, there is the well-known differentiation of leaf surfaces
and of chlorenchym, but this has no relation to the angle of incidence.
Table of Intensity at Various Angles
ANGLE INTENSITY
90 1.00
80 .98
70 .94
60 .87
50 .77
40 .64
30 .50
20 .34
10 .17
In the sunlight, it makes no difference at what height a light reading is taken. In forest
and thicket as well as in some herbaceous formations, the intensity of the light, if there is
any difference, is greatest just beneath the foliage of the facies. In forests especially, the
light is increasingly diffuse toward the ground, particularly where layers intervene. In
woodland formations, moreover, the exact spot in which a reading is made must be
carefully chosen, unless the foliage is so dense that the shade is uniform. A very
satisfactory plan is to take readings in two or more spots where the shade appears to be
typical, and to make a check reading in a “sunfleck,” a spot where sunlight shows
through. In forests and thickets, the sunflecks are fleeting, and the light value is
practically that of the shade. In passing into open woodland and thicket, the sunflecks
increase in size and permanence, until finally they exceed the shade areas in amount and
become typical of the formation.
Reflected and Absorbed Light
88. The fate of incident light. The light present in a habitat and incident upon a leaf is
not all available for photosynthesis. Part is reflected or screened out by the epidermis,
and a certain amount passes through the chlorenchym, except in very thick leaves. The
light absorbed is by far the greatest in the majority of species. Many plants with dense
coatings of hairs reflect or withhold more light than they absorb, and the amount of light
reflected by a thick cuticule is likewise great. As light is imponderable, the actual amount
absorbed or reflected by the leaf can not be determined. It is possible, however, to
Page 90
express this in terms of the total amount received, by means of readings with solio paper,
and the knowledge thus obtained is of great importance in interpreting the modifications
of certain types of leaves. For example, a leaf with a densely hairy epidermis may
receive light of the full intensity, 1; the amount reflected or screened out by the hairs may
be 95 per cent of this, the amount absorbed 5 per cent, and that transmitted, nil. In the
majority of cases, however, the absorbed light is considerably more than the amount
reflected or transmitted.
89. Methods of determination. If
results are to be of value, reflected and
transmitted light must be determined in
the habitat of the plant simultaneously
with the total light which a leaf receives.
An approximation of the light reflected
from a leaf surface is secured by placing
the photometer so that the light reflected
is thrown upon the solio strip. A much
more satisfactory method, however, is to
determine it in connection with the
amount of light transmitted through the
epidermis. This is done by stripping a
piece of epidermis from the upper surface
of the leaf and placing it over the slit in
the photometer for an exposure. An
exposure in the full light of the habitat is
made simultaneously with another
photometer, or immediately afterward
upon the same strip. When the epidermis
is not too dense, both exposures are Fig. 14. Leaf print: exposed 10 m., 11 A.M.
permitted to reach the same tint, and the August 20. The leaves are from sun and
relation between them is precisely that of shade forms of Bursa bursa-pastoris, Rosa
their lengths of exposure. Ordinarily the sayii, Thalictrum sparsiflorum, and
two exposures are made absolutely Machaeranthera aspera. In each the shade
simultaneous by placing the epidermis leaf prints more deeply.
over half of the opening, leaving the other
half to record the full light value, and the
results, or epidermis prints, are referred to a multiple standard. The difference between
the two values thus obtained represents the amount of reflected light together with that
screened by the epidermis. The amount of light transmitted through the leaf may be
measured in the same way by using the leaf itself in place of the epidermis alone. The
time of exposure is necessarily long, however, and it has been found practicable to obtain
leaf prints by exposing the leaf in a printing frame, upon solio paper, at the same time
that the epidermis print is made. In a few species both the upper and lower epidermis can
be removed and the amount of light absorbed determined directly by exposing the strip
and the knowledge thus obtained is of great importance in interpreting the modifications
of certain types of leaves. For example, a leaf with a densely hairy epidermis may
receive light of the full intensity, 1; the amount reflected or screened out by the hairs may
be 95 per cent of this, the amount absorbed 5 per cent, and that transmitted, nil. In the
majority of cases, however, the absorbed light is considerably more than the amount
reflected or transmitted.
89. Methods of determination. If
results are to be of value, reflected and
transmitted light must be determined in
the habitat of the plant simultaneously
with the total light which a leaf receives.
An approximation of the light reflected
from a leaf surface is secured by placing
the photometer so that the light reflected
is thrown upon the solio strip. A much
more satisfactory method, however, is to
determine it in connection with the
amount of light transmitted through the
epidermis. This is done by stripping a
piece of epidermis from the upper surface
of the leaf and placing it over the slit in
the photometer for an exposure. An
exposure in the full light of the habitat is
made simultaneously with another
photometer, or immediately afterward
upon the same strip. When the epidermis
is not too dense, both exposures are Fig. 14. Leaf print: exposed 10 m., 11 A.M.
permitted to reach the same tint, and the August 20. The leaves are from sun and
relation between them is precisely that of shade forms of Bursa bursa-pastoris, Rosa
their lengths of exposure. Ordinarily the sayii, Thalictrum sparsiflorum, and
two exposures are made absolutely Machaeranthera aspera. In each the shade
simultaneous by placing the epidermis leaf prints more deeply.
over half of the opening, leaving the other
half to record the full light value, and the
results, or epidermis prints, are referred to a multiple standard. The difference between
the two values thus obtained represents the amount of reflected light together with that
screened by the epidermis. The amount of light transmitted through the leaf may be
measured in the same way by using the leaf itself in place of the epidermis alone. The
time of exposure is necessarily long, however, and it has been found practicable to obtain
leaf prints by exposing the leaf in a printing frame, upon solio paper, at the same time
that the epidermis print is made. In a few species both the upper and lower epidermis can
be removed and the amount of light absorbed determined directly by exposing the strip
Page 91
covered with the chlorenchym. Generally, however, this must be computed by
subtracting the sum of the per cents of reflected and transmitted light from 100 per cent,
which represents the total light.
90. Leaf and epidermis prints. In
diphotic leaves the screening effect of the
lower epidermis may be ignored.
Isophotic sun leaves, i. e., those nearly
upright in position or found above light-
colored, reflecting soils, are usually
strongly illuminated on both sides, and
the absorbed light can be obtained only
by measuring the screening effect of both
epiderms. Shade leaves and submerged
leaves often contain chloroplasts in the
epidermis, and the above method can not
be applied to them. In fact, in habitats
where the light is quite diffuse, practically
all incident light is absorbed. The rare
exceptions are those shade leaves with a
distinct bloom. In addition to their use in
obtaining the amount of light absorbed,
both leaf and epidermis prints are
extremely interesting for the direct
comparison of light relations in the leaves
of species belonging to different habitats.
The relative screening value of the upper
Fig. 15. Leaf print: exposure as before. Sun
and lower epidermis, or of the
and shade leaves of Achillea lanulosa,
corresponding epiderms of two ecads or
Capnoides aureum, Antennaria umbrinella,
two species, is readily ascertained by
Galium boreale, and Potentilla propinqua.
exposing the two side by side in sunshine,
over the slit in the photometer. For leaf
prints fresh leaves are desirable, though nearly the same results can be obtained from
leaves dried under pressure. The leaves are grouped as desired on the glass of a printing
frame, and covered with a sheet of solio. They are then exposed to full sunlight,
preferably at meridian, and the prints evaluated by means of the multiple standard. This
method is especially useful in the comparison of ecads of one species. These differences
due to transmitted light are very graphic, and can easily be preserved by “toning” the
print in the usual way.
Expression of Results
subtracting the sum of the per cents of reflected and transmitted light from 100 per cent,
which represents the total light.
90. Leaf and epidermis prints. In
diphotic leaves the screening effect of the
lower epidermis may be ignored.
Isophotic sun leaves, i. e., those nearly
upright in position or found above light-
colored, reflecting soils, are usually
strongly illuminated on both sides, and
the absorbed light can be obtained only
by measuring the screening effect of both
epiderms. Shade leaves and submerged
leaves often contain chloroplasts in the
epidermis, and the above method can not
be applied to them. In fact, in habitats
where the light is quite diffuse, practically
all incident light is absorbed. The rare
exceptions are those shade leaves with a
distinct bloom. In addition to their use in
obtaining the amount of light absorbed,
both leaf and epidermis prints are
extremely interesting for the direct
comparison of light relations in the leaves
of species belonging to different habitats.
The relative screening value of the upper
Fig. 15. Leaf print: exposure as before. Sun
and lower epidermis, or of the
and shade leaves of Achillea lanulosa,
corresponding epiderms of two ecads or
Capnoides aureum, Antennaria umbrinella,
two species, is readily ascertained by
Galium boreale, and Potentilla propinqua.
exposing the two side by side in sunshine,
over the slit in the photometer. For leaf
prints fresh leaves are desirable, though nearly the same results can be obtained from
leaves dried under pressure. The leaves are grouped as desired on the glass of a printing
frame, and covered with a sheet of solio. They are then exposed to full sunlight,
preferably at meridian, and the prints evaluated by means of the multiple standard. This
method is especially useful in the comparison of ecads of one species. These differences
due to transmitted light are very graphic, and can easily be preserved by “toning” the
print in the usual way.
Expression of Results
Page 92
91. Light records. The actual photographic records obtained by photometer and
selagraph can at most be kept but a few months, unless they are toned or fixed. “Toning”
modifies the color of the exposure materially, and changes its intensity so that it can not
be compared with readings not fixed. It would involve a great deal of inconvenience to
make all comparisons by means of toned strips and standard, even if it were not for the
fact that it is practically impossible to obtain exactly the same shade in lots toned at
different times. The field record, if carefully and neatly made, may well take the place of
a permanent one. The form is the following:
selagraph can at most be kept but a few months, unless they are toned or fixed. “Toning”
modifies the color of the exposure materially, and changes its intensity so that it can not
be compared with readings not fixed. It would involve a great deal of inconvenience to
make all comparisons by means of toned strips and standard, even if it were not for the
fact that it is practically impossible to obtain exactly the same shade in lots toned at
different times. The field record, if carefully and neatly made, may well take the place of
a permanent one. The form is the following:
Page 93
Day Hour Formation Station Altitude Exposure Group Height No. Length of Standard Light Base Reflected Transm’d Absorbed
exposure value value light light light
14/9/04 12:00 Spruce Milky 2600 N.E. Opulaster 1 foot 2:10 160 s. 3s .019
M. Way m. 20°
„ 12:05 Spruce Moss 2500 Level Streptopus „ 2:12 240 s. 3 s. .012
P.M. Glen m.
„ 12:15 Brook Grotto 2500 E. 3° Filix Surface 2:13 360 s. 3 s. .008
P.M. b’nk m.
92. Light sums, means, and curves. Owing to the fact that the selagraph has not yet
been used in the field, no endeavor has been made to determine the light value for every
hour of the day in different habitats. Consequently there has been no attempt to compute
light sums and means. Photometer readings have sufficed to interpret the effect of light in
the structure of the formation, and of the individual, but they have not been sufficiently
frequent for use in ascertaining sums and means. The latter are much less valuable than
the extremes, especially when the relative duration of these is indicated. Means,
however, are readily obtained from the continuous records. Light sums are probably
impracticable, as the factor is not one that can be expressed in absolute terms. The
various kinds and combinations of light curves are essentially the same as for humidity.
The level curve through a series of habitats is the most illuminating, but the day curve of
hour variations is of considerable value. The curve of daily duration, based upon full
sunlight, is also of especial importance for plants, and stations which receive both sun
and shade during the day.
exposure value value light light light
14/9/04 12:00 Spruce Milky 2600 N.E. Opulaster 1 foot 2:10 160 s. 3s .019
M. Way m. 20°
„ 12:05 Spruce Moss 2500 Level Streptopus „ 2:12 240 s. 3 s. .012
P.M. Glen m.
„ 12:15 Brook Grotto 2500 E. 3° Filix Surface 2:13 360 s. 3 s. .008
P.M. b’nk m.
92. Light sums, means, and curves. Owing to the fact that the selagraph has not yet
been used in the field, no endeavor has been made to determine the light value for every
hour of the day in different habitats. Consequently there has been no attempt to compute
light sums and means. Photometer readings have sufficed to interpret the effect of light in
the structure of the formation, and of the individual, but they have not been sufficiently
frequent for use in ascertaining sums and means. The latter are much less valuable than
the extremes, especially when the relative duration of these is indicated. Means,
however, are readily obtained from the continuous records. Light sums are probably
impracticable, as the factor is not one that can be expressed in absolute terms. The
various kinds and combinations of light curves are essentially the same as for humidity.
The level curve through a series of habitats is the most illuminating, but the day curve of
hour variations is of considerable value. The curve of daily duration, based upon full
sunlight, is also of especial importance for plants, and stations which receive both sun
and shade during the day.
Page 94
TEMPERATURE
93. In consequence of its indirect action, temperature does not have a striking effect
upon the form and structure of the plant, as is the case with water and light.
Notwithstanding, it is a factor of fundamental importance. This is especially evident in
the character and distribution of vegetation. It is also seen in the germination and growth
of plants, in the length of season, and in the important influence of temperature upon
humidity, and hence upon water-content. Because of its intimate relation with the
comfort of mankind, the determination of temperature values has received more attention
than that of any other factor, and excellent simple and recording instruments are
numerous. For plants, it is also necessary to employ instruments for measuring soil
temperatures. The latter unquestionably have much less meaning for the plant than the
temperatures of the air, but they have a direct influence upon the imbibition of water, and
upon germination.
Thermometers
94. Air thermometers. The accurate measurement of temperature requires standard
thermometers. Reasonably accurate instruments may be standardized by determining
their error, but they are extremely unsatisfactory in practice, since they result in a serious
waste of time. Accurate thermometers which read to the degree are entirely serviceable
as a rule, but instruments which read to a fraction of a degree are often very much to be
desired. The writer has found the “cylindrical bulb thermometer, Centigrade scale” of H.
J. Green, to be an exceedingly satisfactory instrument. The best numbers for general use
are 247 and 251, which read from –15° to 50° C. and are graduated in .2°. They are
respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are
delicate and require careful handling, but even in class work this has proved to be an
advantage rather than otherwise. In making readings of air temperatures with such
thermometers, constant precautions must be taken to expose the bulb directly to the wind
and to keep it away from the hand and person.
95. Soil thermometers. The thermometer described above has been used extensively
for soil temperatures. The determination of the latter is conveniently combined with the
taking of soil samples, by using the hole for a temperature reading. When carefully
covered, these holes can be used from day to day throughout the season without
appreciable error, even in gravel soils. Repeated tests of this have been made by
simultaneous readings in permanent and newly made holes, and the results have always
been the same. It has even been found that the error is usually less than 1 degree when
the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in
the fact that the thermometer must be raised to make the reading. With a little practice,
however, the top of the column of mercury may be raised to the surface and read before
93. In consequence of its indirect action, temperature does not have a striking effect
upon the form and structure of the plant, as is the case with water and light.
Notwithstanding, it is a factor of fundamental importance. This is especially evident in
the character and distribution of vegetation. It is also seen in the germination and growth
of plants, in the length of season, and in the important influence of temperature upon
humidity, and hence upon water-content. Because of its intimate relation with the
comfort of mankind, the determination of temperature values has received more attention
than that of any other factor, and excellent simple and recording instruments are
numerous. For plants, it is also necessary to employ instruments for measuring soil
temperatures. The latter unquestionably have much less meaning for the plant than the
temperatures of the air, but they have a direct influence upon the imbibition of water, and
upon germination.
Thermometers
94. Air thermometers. The accurate measurement of temperature requires standard
thermometers. Reasonably accurate instruments may be standardized by determining
their error, but they are extremely unsatisfactory in practice, since they result in a serious
waste of time. Accurate thermometers which read to the degree are entirely serviceable
as a rule, but instruments which read to a fraction of a degree are often very much to be
desired. The writer has found the “cylindrical bulb thermometer, Centigrade scale” of H.
J. Green, to be an exceedingly satisfactory instrument. The best numbers for general use
are 247 and 251, which read from –15° to 50° C. and are graduated in .2°. They are
respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are
delicate and require careful handling, but even in class work this has proved to be an
advantage rather than otherwise. In making readings of air temperatures with such
thermometers, constant precautions must be taken to expose the bulb directly to the wind
and to keep it away from the hand and person.
95. Soil thermometers. The thermometer described above has been used extensively
for soil temperatures. The determination of the latter is conveniently combined with the
taking of soil samples, by using the hole for a temperature reading. When carefully
covered, these holes can be used from day to day throughout the season without
appreciable error, even in gravel soils. Repeated tests of this have been made by
simultaneous readings in permanent and newly made holes, and the results have always
been the same. It has even been found that the error is usually less than 1 degree when
the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in
the fact that the thermometer must be raised to make the reading. With a little practice,
however, the top of the column of mercury may be raised to the surface and read before
Page 95
the change of temperature can react upon it. This is especially important in very moist or
wet soils where the bulb becomes coated with a film of moisture. This evaporates when
the bulb is brought into the air, and after a moment or two the mercury slowly falls.
wet soils where the bulb becomes coated with a film of moisture. This evaporates when
the bulb is brought into the air, and after a moment or two the mercury slowly falls.
Page 96
Regular soil thermometers are indispensable when readings are desired at
depths greater than 12–18 inches. They possess several disadvantages
which restrict their use almost wholly to permanent stations. It is scarcely
possible to carry them on field trips, and the time required to place them in
the soil renders them practically useless for single readings. Moreover, the
instruments are expensive, ranging in price from $7 for the two-foot
thermometer, to $19 for the eight-foot one. When it is recognized that deep-
seated temperatures are extremely constant and that the slight fluctuations
affect, as a rule, only the relatively stable shrubs and trees, it is evident that
such temperatures are of restricted importance. Still, in any habitat, they
must be ascertained before they can well be ignored, though it is unwise to
spend much time and energy in their determination. Soil thermometers of
the form illustrated may be obtained from H. J. Green, Brooklyn.
96. Maximum-minimum thermometers. These are used for
determining the range of temperature within a given period, usually a day.
Since they are much cheaper than thermographs, they can replace these in
part, although they merely indicate the maximum and minimum
temperatures for the day, and do not register the time when each occurs.
The maximum is a mercurial thermometer with a constriction in the tube
just above the bulb; this allows the mercury to pass out as it expands, but
prevents it from running back, thus registering the maximum temperature.
The minimum thermometer contains alcohol. The column carries a tiny
dumbbell-shaped marker which moves down with it, but will not rise as the
liquid expands. This is due to the fact that the fluid expands too slowly to
carry the marker upward, while the surface tension causes it to be drawn
downward as the fluid contracts. The minimum temperature is indicated by
the upper end of the marker. In setting up the thermometers, they are
attached by special thumbscrews to a support which holds them in an
oblique position. The minimum is placed in a special holder above the
maximum which rests on a pin that is used also for screwing the pivot-
screw into position. The support is screwed tightly to the cross-piece of a
post, or in forest formations it is fastened directly to a board nailed upon a
tree trunk. A shelter has not been used in ecological work, although it is the
rule in meteorological observations. The minimum thermometer is set for
registering by raising the free end, so that the marker runs to the end of the
column. The mercury of the maximum is driven back into the bulb by
whirling it rapidly on the pivot-screw after the pin has been taken out. This
depths greater than 12–18 inches. They possess several disadvantages
which restrict their use almost wholly to permanent stations. It is scarcely
possible to carry them on field trips, and the time required to place them in
the soil renders them practically useless for single readings. Moreover, the
instruments are expensive, ranging in price from $7 for the two-foot
thermometer, to $19 for the eight-foot one. When it is recognized that deep-
seated temperatures are extremely constant and that the slight fluctuations
affect, as a rule, only the relatively stable shrubs and trees, it is evident that
such temperatures are of restricted importance. Still, in any habitat, they
must be ascertained before they can well be ignored, though it is unwise to
spend much time and energy in their determination. Soil thermometers of
the form illustrated may be obtained from H. J. Green, Brooklyn.
96. Maximum-minimum thermometers. These are used for
determining the range of temperature within a given period, usually a day.
Since they are much cheaper than thermographs, they can replace these in
part, although they merely indicate the maximum and minimum
temperatures for the day, and do not register the time when each occurs.
The maximum is a mercurial thermometer with a constriction in the tube
just above the bulb; this allows the mercury to pass out as it expands, but
prevents it from running back, thus registering the maximum temperature.
The minimum thermometer contains alcohol. The column carries a tiny
dumbbell-shaped marker which moves down with it, but will not rise as the
liquid expands. This is due to the fact that the fluid expands too slowly to
carry the marker upward, while the surface tension causes it to be drawn
downward as the fluid contracts. The minimum temperature is indicated by
the upper end of the marker. In setting up the thermometers, they are
attached by special thumbscrews to a support which holds them in an
oblique position. The minimum is placed in a special holder above the
maximum which rests on a pin that is used also for screwing the pivot-
screw into position. The support is screwed tightly to the cross-piece of a
post, or in forest formations it is fastened directly to a board nailed upon a
tree trunk. A shelter has not been used in ecological work, although it is the
rule in meteorological observations. The minimum thermometer is set for
registering by raising the free end, so that the marker runs to the end of the
column. The mercury of the maximum is driven back into the bulb by
whirling it rapidly on the pivot-screw after the pin has been taken out. This
Page 97
must be done with care in order
that the bulb may not be broken.
As soon as the instrument comes to
rest, it is raised and the pin
replaced, great care being taken to
lift it no higher than is necessary.
When the night maximum is
sought, the thermometer should be
whirled several times in order to
drive the column sufficiently low.
Usually, in such cases, a record is
made of this point to make sure
that the maximum read is the
actual one. If the pivot-screw is
kept well oiled, less force will be
required to drive the mercury back.
In practice, the thermometers have
been observed at 6:00 A.M. and
6:00 P.M. each day, thus permitting
the reading of the maximum-
minimum for both day and night.
Pairs of maximum-minimum
thermometers are to be obtained
from H. J. Green, 1191 Bedford
Ave., Brooklyn, or Julien P. Friez,
Baltimore, Maryland, at a cost of
$8.25.
97. Radiation thermometers.
These are used to determine the
radiation in the air, and from the
soil, i. e., for solar and terrestrial
radiation. The latter alone has been
employed in the study of habitats,
chiefly for the purpose of
ascertaining the difference in the
cooling of different soils at night.
The terrestrial radiation
that the bulb may not be broken.
As soon as the instrument comes to
rest, it is raised and the pin
replaced, great care being taken to
lift it no higher than is necessary.
When the night maximum is
sought, the thermometer should be
whirled several times in order to
drive the column sufficiently low.
Usually, in such cases, a record is
made of this point to make sure
that the maximum read is the
actual one. If the pivot-screw is
kept well oiled, less force will be
required to drive the mercury back.
In practice, the thermometers have
been observed at 6:00 A.M. and
6:00 P.M. each day, thus permitting
the reading of the maximum-
minimum for both day and night.
Pairs of maximum-minimum
thermometers are to be obtained
from H. J. Green, 1191 Bedford
Ave., Brooklyn, or Julien P. Friez,
Baltimore, Maryland, at a cost of
$8.25.
97. Radiation thermometers.
These are used to determine the
radiation in the air, and from the
soil, i. e., for solar and terrestrial
radiation. The latter alone has been
employed in the study of habitats,
chiefly for the purpose of
ascertaining the difference in the
cooling of different soils at night.
The terrestrial radiation
Page 98
thermometer is merely a special
form of minimum thermometer, so
arranged in a support that the bulb
can be placed directly above the
soil or plant to be studied. It is
otherwise operated exactly like the
minimum thermometer, and the
reading gives the minimum
temperature which the air above
the plant or soil reaches, not the
amount of radiation. As a
consequence, these instruments are
valuable only where read in
connection with a pair of
maximum-minimum thermometers
in the air, or when read in a series
of instruments placed above
different soils or plants.
98. Thermographs. Two types
of standard instruments are in
general use for obtaining
continuous records of air
temperatures. These are the Draper
thermograph, made by the Draper
Manufacturing Company, 152
Front St., New York city ($25 and
$30), and the Richard thermograph
sold by Julien P. Friez, Baltimore
($50). After careful trial had
demonstrated that they were
equally accurate, the matter of cost
was considered decisive, and the
Draper thermograph has been used
exclusively in the writer’s own
work. This instrument closely
resembles the psychrograph
manufactured by the same
form of minimum thermometer, so
arranged in a support that the bulb
can be placed directly above the
soil or plant to be studied. It is
otherwise operated exactly like the
minimum thermometer, and the
reading gives the minimum
temperature which the air above
the plant or soil reaches, not the
amount of radiation. As a
consequence, these instruments are
valuable only where read in
connection with a pair of
maximum-minimum thermometers
in the air, or when read in a series
of instruments placed above
different soils or plants.
98. Thermographs. Two types
of standard instruments are in
general use for obtaining
continuous records of air
temperatures. These are the Draper
thermograph, made by the Draper
Manufacturing Company, 152
Front St., New York city ($25 and
$30), and the Richard thermograph
sold by Julien P. Friez, Baltimore
($50). After careful trial had
demonstrated that they were
equally accurate, the matter of cost
was considered decisive, and the
Draper thermograph has been used
exclusively in the writer’s own
work. This instrument closely
resembles the psychrograph
manufactured by the same
Page 99
company. It is made in two sizes, Fig. 16. Soil thermometer
of which the larger one is the more
satisfactory on account of the
greater distance between the lines
of the recording disk. The
thermometric part consists of two
bimetallic strips, the contraction
Fig. 17. Maximum-minimum and expansion of which are
thermometer. communicated to a hand carrying a
pen. The latter traces a line on the
record sheet which is attached to a
metal disk made to revolve by an
eight-day clock. In practice the
thermograph is set up in the shelter
which contains the psychrograph,
and in exactly the same manner.
The clock is wound, the record put
in place, and the pen inked in the
same way also. The proper position
of the pen is determined by making Fig. 18. Terrestrial radiation
a careful thermometer reading thermometer.
under the shelter, and then
regulating the pen hand by means
of the screws at the base of it. A similar test reading is also made each
week, when the clock is rewound. A record sheet may be left in position for
three weeks, the pen being filled each week with a different ink. The fixed
order of using the inks is red, blue, and green as already indicated.
Owing to the fact that they are practically stationary, soil thermographs
are of slight value, except at base stations. Here, the facts that they are
expensive, that the soil temperatures are of relatively little importance, and
that they can be determined as easily, or nearly so, by simple thermometers,
make the use of such instruments altogether unnecessary, if not, indeed,
undesirable. In a perfectly equipped research station, they undoubtedly have
their use, but at ordinary stations, and in the case of private investigators,
their value is in no wise commensurate with their cost.
of which the larger one is the more
satisfactory on account of the
greater distance between the lines
of the recording disk. The
thermometric part consists of two
bimetallic strips, the contraction
Fig. 17. Maximum-minimum and expansion of which are
thermometer. communicated to a hand carrying a
pen. The latter traces a line on the
record sheet which is attached to a
metal disk made to revolve by an
eight-day clock. In practice the
thermograph is set up in the shelter
which contains the psychrograph,
and in exactly the same manner.
The clock is wound, the record put
in place, and the pen inked in the
same way also. The proper position
of the pen is determined by making Fig. 18. Terrestrial radiation
a careful thermometer reading thermometer.
under the shelter, and then
regulating the pen hand by means
of the screws at the base of it. A similar test reading is also made each
week, when the clock is rewound. A record sheet may be left in position for
three weeks, the pen being filled each week with a different ink. The fixed
order of using the inks is red, blue, and green as already indicated.
Owing to the fact that they are practically stationary, soil thermographs
are of slight value, except at base stations. Here, the facts that they are
expensive, that the soil temperatures are of relatively little importance, and
that they can be determined as easily, or nearly so, by simple thermometers,
make the use of such instruments altogether unnecessary, if not, indeed,
undesirable. In a perfectly equipped research station, they undoubtedly have
their use, but at ordinary stations, and in the case of private investigators,
their value is in no wise commensurate with their cost.
Page 100
Readings
99. Time. The hourly and daily
fluctuations of the temperature of
the air render frequent readings
desirable. It is this variation,
indeed, which makes single
readings, or even series of them,
inconclusive, and renders the use
of a recording instrument almost
imperative in the base station at
least. Undoubtedly, a set of
simultaneous readings at different
heights in one station, or at the
same height in different stations,
especially if made at the
maximum, have much value for
comparison, but their full
significance is seen only when they
Fig. 19. Draper thermograph. are referred to a continuous base
record. Such series, moreover,
furnish good results for purposes of instruction. In research work, however,
it has been found imperative to have thermographs in habitats of widely
different character. With these as bases, it is possible to eke them out with
considerable satisfaction by means of maximum-minimum thermometers in
less different habitats, or in different parts of the same habitat. Naturally
these are less satisfactory, and are used only when expense sets a limit to
the number of thermographs. In a careful analysis of a single habitat, more
can be gained by one base thermograph supplemented by three pairs of
maximum-minimum thermometers in dissimilar areas of the habitat than by
two thermographs, and the cost is the same.
99. Time. The hourly and daily
fluctuations of the temperature of
the air render frequent readings
desirable. It is this variation,
indeed, which makes single
readings, or even series of them,
inconclusive, and renders the use
of a recording instrument almost
imperative in the base station at
least. Undoubtedly, a set of
simultaneous readings at different
heights in one station, or at the
same height in different stations,
especially if made at the
maximum, have much value for
comparison, but their full
significance is seen only when they
Fig. 19. Draper thermograph. are referred to a continuous base
record. Such series, moreover,
furnish good results for purposes of instruction. In research work, however,
it has been found imperative to have thermographs in habitats of widely
different character. With these as bases, it is possible to eke them out with
considerable satisfaction by means of maximum-minimum thermometers in
less different habitats, or in different parts of the same habitat. Naturally
these are less satisfactory, and are used only when expense sets a limit to
the number of thermographs. In a careful analysis of a single habitat, more
can be gained by one base thermograph supplemented by three pairs of
maximum-minimum thermometers in dissimilar areas of the habitat than by
two thermographs, and the cost is the same.
Page 101
Fig. 20. Shelter for thermograph.
Fig. 21. Richard thermograph.
Fig. 21. Richard thermograph.
Page 102
100. Place and height. For general air temperatures, thermograph and
thermometer readings are made at a height of 3 feet (1 meter). Soil
temperatures are regularly taken at the surface and at a depth of 1 foot.
When a complete series of simultaneous readings is made in one station, the
levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 10, and
15 inches in the soil. When sun and shade occur side by side in the same
formation, as is true of many thickets and forests, surface readings are
regularly made in both. Similarly, valuable results are obtained by making
simultaneous readings on the bare soil, on dead cover, and upon a leaf,
while the influence of cover is readily ascertained by readings upon it and
beneath it. A full series of station readings made at the same time upon
north, east, south, and west slopes is of great importance in studying the
effects of exposure.
Expression of Results
101. Temperature records. Neither field nor permanent form is required
for thermographic records, other than the record sheet itself, which contains
all the necessary information in a fairly convenient form. Although the
temperature of a particular hour and day can not be read at a mere glance, it
can be obtained so easily that it is a waste of time to make a tabular copy of
each record sheet. For thermometer readings, either single or in series, the
following form is used:
thermometer readings are made at a height of 3 feet (1 meter). Soil
temperatures are regularly taken at the surface and at a depth of 1 foot.
When a complete series of simultaneous readings is made in one station, the
levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 10, and
15 inches in the soil. When sun and shade occur side by side in the same
formation, as is true of many thickets and forests, surface readings are
regularly made in both. Similarly, valuable results are obtained by making
simultaneous readings on the bare soil, on dead cover, and upon a leaf,
while the influence of cover is readily ascertained by readings upon it and
beneath it. A full series of station readings made at the same time upon
north, east, south, and west slopes is of great importance in studying the
effects of exposure.
Expression of Results
101. Temperature records. Neither field nor permanent form is required
for thermographic records, other than the record sheet itself, which contains
all the necessary information in a fairly convenient form. Although the
temperature of a particular hour and day can not be read at a mere glance, it
can be obtained so easily that it is a waste of time to make a tabular copy of
each record sheet. For thermometer readings, either single or in series, the
following form is used:
Page 103
Day Hour Formation Station Altitude Exposure Community POSITION OF READING Thermograph Sky Wind
3 feet Surf. 12 in.
17/8/04 6:30 Spruce Jack 2550 N.E. Mertensiare 9° 9° 9.8° 10° Clear 0
A.M. Brook m. 5°
„ „ Half Hiawatha 2550 N.E. Asterare 11.2° 11.2° 14.8° 10° Clear 0
gravel m. 7°
„ 6:30 Spruce Jack 2550 N.E. Mertensiare 11.4° 11.4° 9.8° 11° Cloudy 0
P.M. Brook m. 5°
„ „ Half Hiawatha 2550 N.E. Asterare 12° 13.8° 16.4° 11° Cloudy 0
gravel m. 7°
102. Temperature sums and means. The amount of heat, i. e., the
number of calories received within a given time by a definite area of plant
surface, can be determined by means of a calorimeter. From this the
temperature sum of a particular period may be obtained by simple addition.
In the present condition of our knowledge, it is impossible to establish any
exact connection between such results and the functional or growth effect
that can be traced directly to heat. As a consequence, temperature sums do
not at present contribute anything of value to an understanding of the
relation between cause and effect. The mean daily temperature is readily
obtained by averaging twenty-four hour-temperatures recorded by the
thermograph. The method employed by Meyen[6], of deriving the mean
directly from the maximum and minimum for the day, is not accurate; from
a large number of computations, the error is always more than two degrees.
On the other hand, the mean obtained by averaging the maximum and
minimum for the day and night has been found to deviate less than 1 degree
from the mean proper. This fact greatly increases the value of maximum-
minimum instruments if they are read daily at 6:00 A.M. and 6:00 P.M.
103. Temperature curves. The kinds and combinations of temperature
curves are almost without number. The simple curves of most interest are
those for a series of stations or habitats, based upon the level of three feet,
or the surface, or the daily mean. The curves for each station representing
the different heights and depths and the season curve of the daily means for
a habitat are also of much importance. One of the most illuminating
combinations is that which groups together the various level curves for a
series of habitats. Other valuable combinations are obtained by grouping the
curves of daily means of different habitats for the season, or the various
station curves.
104. Plant temperatures. The direct effects of temperature as seen in
nutrition and growth can be ascertained only by determining the
temperature of plant tissues. The temperatures of the air and of the soil
3 feet Surf. 12 in.
17/8/04 6:30 Spruce Jack 2550 N.E. Mertensiare 9° 9° 9.8° 10° Clear 0
A.M. Brook m. 5°
„ „ Half Hiawatha 2550 N.E. Asterare 11.2° 11.2° 14.8° 10° Clear 0
gravel m. 7°
„ 6:30 Spruce Jack 2550 N.E. Mertensiare 11.4° 11.4° 9.8° 11° Cloudy 0
P.M. Brook m. 5°
„ „ Half Hiawatha 2550 N.E. Asterare 12° 13.8° 16.4° 11° Cloudy 0
gravel m. 7°
102. Temperature sums and means. The amount of heat, i. e., the
number of calories received within a given time by a definite area of plant
surface, can be determined by means of a calorimeter. From this the
temperature sum of a particular period may be obtained by simple addition.
In the present condition of our knowledge, it is impossible to establish any
exact connection between such results and the functional or growth effect
that can be traced directly to heat. As a consequence, temperature sums do
not at present contribute anything of value to an understanding of the
relation between cause and effect. The mean daily temperature is readily
obtained by averaging twenty-four hour-temperatures recorded by the
thermograph. The method employed by Meyen[6], of deriving the mean
directly from the maximum and minimum for the day, is not accurate; from
a large number of computations, the error is always more than two degrees.
On the other hand, the mean obtained by averaging the maximum and
minimum for the day and night has been found to deviate less than 1 degree
from the mean proper. This fact greatly increases the value of maximum-
minimum instruments if they are read daily at 6:00 A.M. and 6:00 P.M.
103. Temperature curves. The kinds and combinations of temperature
curves are almost without number. The simple curves of most interest are
those for a series of stations or habitats, based upon the level of three feet,
or the surface, or the daily mean. The curves for each station representing
the different heights and depths and the season curve of the daily means for
a habitat are also of much importance. One of the most illuminating
combinations is that which groups together the various level curves for a
series of habitats. Other valuable combinations are obtained by grouping the
curves of daily means of different habitats for the season, or the various
station curves.
104. Plant temperatures. The direct effects of temperature as seen in
nutrition and growth can be ascertained only by determining the
temperature of plant tissues. The temperatures of the air and of the soil
Page 104
surface have an important effect upon humidity, and water-content, and
through them upon the plant, but heat can influence assimilation, for
example, only in so far as it is absorbed by the assimilating tissue. The
temperatures of the leaf, as the most active nutritive organ of the plant, are
especially important. While it is a well-known fact that internal
temperatures follow those of the air and soil closely, though with varying
rapidity of response, this holds less for leaves than for stems and roots.
Owing to the very obvious difficulties, practically nothing has yet been
done in this important field. A few preliminary results have been obtained at
Minnehaha, which serve to show the need for such readings. Gravel slide
rosettes in an air temperature of 24° C. and a surface temperature of 40° C.
gave the following surface readings: Parmelia, 40°, Eriogonum, 38.6°,
Arctostaphylus, 35°, Thlaspi, 31.8°, and Senecio, 31°. The leaf of
Eriogonum flavum, which is smooth above and densely hairy below,
indicated a temperature of 31.8° when rolled closely about the thermometer
bulb with the smooth surface out, and 28° when the hairy surface was
outside. The surface readings of the same leaf were .5°–1° higher when
made upon the upper smooth surface. This immediately suggests that the
lower surface may be modified to protect the leaf from the great heat of the
gravel, which often reaches 50° C. (122° F.).
through them upon the plant, but heat can influence assimilation, for
example, only in so far as it is absorbed by the assimilating tissue. The
temperatures of the leaf, as the most active nutritive organ of the plant, are
especially important. While it is a well-known fact that internal
temperatures follow those of the air and soil closely, though with varying
rapidity of response, this holds less for leaves than for stems and roots.
Owing to the very obvious difficulties, practically nothing has yet been
done in this important field. A few preliminary results have been obtained at
Minnehaha, which serve to show the need for such readings. Gravel slide
rosettes in an air temperature of 24° C. and a surface temperature of 40° C.
gave the following surface readings: Parmelia, 40°, Eriogonum, 38.6°,
Arctostaphylus, 35°, Thlaspi, 31.8°, and Senecio, 31°. The leaf of
Eriogonum flavum, which is smooth above and densely hairy below,
indicated a temperature of 31.8° when rolled closely about the thermometer
bulb with the smooth surface out, and 28° when the hairy surface was
outside. The surface readings of the same leaf were .5°–1° higher when
made upon the upper smooth surface. This immediately suggests that the
lower surface may be modified to protect the leaf from the great heat of the
gravel, which often reaches 50° C. (122° F.).
Page 105
PRECIPITATION
105. General relations. As the factor which exerts the most important
control upon water-content and humidity, rainfall must be carefully
considered by the ecologist. It is such an obvious factor, and is usually
spoken of in such general terms that the need of following it accurately is
not evident at once. When it is recognized that the fluctuations of water-
content are directly traceable to it, it becomes clear that its determination is
as important as that of any indirect factor. This does not mean, however,
that the amount of yearly rainfall is to be taken from the records of the
nearest weather station, and the factor dismissed. Like other instruments,
the rain gauge must be kept at the base station of the area under study, and
when this is extensive or diverse, additional instruments should be put into
commission. While the different parts of the same general climatic region
may receive practically the same amount of precipitation during the year, it
is not necessarily true that the rainfall of any particular storm is equally
distributed, especially in the mountains. Nothing less than an exact
knowledge of the amount of rain that falls in the different areas will make it
possible to tell how much of the water-content found at any particular time
in these represents merely the chance differences of precipitation.
The forms of precipitation are rain, dew, hail, snow, and frost. Of these,
hail is too infrequent to be taken into account, while frost usually occurs
only at the extremes of the growing season, and in its effect is rather to be
reckoned with temperature. Snow rarely falls except during the period of
rest, and, while it plays an important part as cover, it is merely one of
several factors that determine the water-content of the soil at the beginning
of spring. The influence of dew is not clearly understood. It is almost
always too slight in amount and too fleeting to affect the water-content of
the soil. It seems probable that it may serve by its own evaporation to
decrease in some degree the water loss from the soil, and from bedewed
plants. If, however, the dew is largely formed by the water of the soil and of
the plant, as is thought by some, then it is negligible as a reinforcement of
water-content. From the above, it is evident that rainfall alone exerts a
profound effect upon the habitat, and it is with its measurement that the
ecologist is chiefly concerned.
105. General relations. As the factor which exerts the most important
control upon water-content and humidity, rainfall must be carefully
considered by the ecologist. It is such an obvious factor, and is usually
spoken of in such general terms that the need of following it accurately is
not evident at once. When it is recognized that the fluctuations of water-
content are directly traceable to it, it becomes clear that its determination is
as important as that of any indirect factor. This does not mean, however,
that the amount of yearly rainfall is to be taken from the records of the
nearest weather station, and the factor dismissed. Like other instruments,
the rain gauge must be kept at the base station of the area under study, and
when this is extensive or diverse, additional instruments should be put into
commission. While the different parts of the same general climatic region
may receive practically the same amount of precipitation during the year, it
is not necessarily true that the rainfall of any particular storm is equally
distributed, especially in the mountains. Nothing less than an exact
knowledge of the amount of rain that falls in the different areas will make it
possible to tell how much of the water-content found at any particular time
in these represents merely the chance differences of precipitation.
The forms of precipitation are rain, dew, hail, snow, and frost. Of these,
hail is too infrequent to be taken into account, while frost usually occurs
only at the extremes of the growing season, and in its effect is rather to be
reckoned with temperature. Snow rarely falls except during the period of
rest, and, while it plays an important part as cover, it is merely one of
several factors that determine the water-content of the soil at the beginning
of spring. The influence of dew is not clearly understood. It is almost
always too slight in amount and too fleeting to affect the water-content of
the soil. It seems probable that it may serve by its own evaporation to
decrease in some degree the water loss from the soil, and from bedewed
plants. If, however, the dew is largely formed by the water of the soil and of
the plant, as is thought by some, then it is negligible as a reinforcement of
water-content. From the above, it is evident that rainfall alone exerts a
profound effect upon the habitat, and it is with its measurement that the
ecologist is chiefly concerned.
Page 106
Fig. 22. Rain gauge showing construction.
106. The rain gauge, as the illustration shows, is a cylindrical vessel
with a funnel-shaped receiver at the top, which is 8 inches in diameter. The
receiver fits closely upon a narrower brass vessel or measuring tube in
which the rain collects. The ratio of surface between receiver and tube is 10
to 1. For readings covering a general area, the rain-gauge is placed in the
open, away from buildings or other obstructions, and is sunken in the
ground sufficiently to keep it upright. In localities where winds are strong, it
is usually braced at the sides also or supported by a wooden frame. In
measuring the amount of rain in the measuring tube, the depth is divided by
ten in order to ascertain the actual rainfall. The depth is measured by
inserting the measuring-rod through the hole in the funnel until it touches
the bottom. It is left for a second or so, quickly withdrawn, and the limit of
106. The rain gauge, as the illustration shows, is a cylindrical vessel
with a funnel-shaped receiver at the top, which is 8 inches in diameter. The
receiver fits closely upon a narrower brass vessel or measuring tube in
which the rain collects. The ratio of surface between receiver and tube is 10
to 1. For readings covering a general area, the rain-gauge is placed in the
open, away from buildings or other obstructions, and is sunken in the
ground sufficiently to keep it upright. In localities where winds are strong, it
is usually braced at the sides also or supported by a wooden frame. In
measuring the amount of rain in the measuring tube, the depth is divided by
ten in order to ascertain the actual rainfall. The depth is measured by
inserting the measuring-rod through the hole in the funnel until it touches
the bottom. It is left for a second or so, quickly withdrawn, and the limit of
Page 107
the wetted portion noted. In the case of standard rods, the actual rainfall is
read directly in hundredths, so that the division by ten is unnecessary. After
each reading, the measuring tube is carefully drained, replaced, and the
receiver put in position. No regular time for making readings is necessary.
During a rainy period, it is customary to make a measurement each day, but
it has been found more satisfactory for ecological purposes to measure each
shower, and to record its duration. These two facts furnish a ready clue to
the relative amount of run-off in each fall of rain. The measurement of
snowfall is often made merely by determining its depth. For comparison
with rainfall, the rain gauge with receiver and tube withdrawn is used. The
snow which falls is melted, poured into the measuring tube, and measured
in the ordinary way. The U. S. Weather Bureau standard rain gauge, with
measuring stick, may be obtained of H. J. Green, or of J. P. Friez for $5.25.
107. Precipitation records. From the periodic character of precipitation,
rainfall sums, means, and curves have little importance in the careful study
of the habitat. The rainfall curve for the growing season is an aid in
explaining the curve of water-content, and the mean rainfall of a region
gives some idea of its vegetation, though even here the matter of its
distribution is of primary importance. The rain and snow charts published
by the U. S. Weather Bureau furnish data of some importance for the
general study of vegetation, but it is evident that they can play little part in a
system which is founded upon the habitat. Precipitation records, for reasons
of brevity and convenience, are united with wind records, and the form will
be found under the discussion of this factor.
read directly in hundredths, so that the division by ten is unnecessary. After
each reading, the measuring tube is carefully drained, replaced, and the
receiver put in position. No regular time for making readings is necessary.
During a rainy period, it is customary to make a measurement each day, but
it has been found more satisfactory for ecological purposes to measure each
shower, and to record its duration. These two facts furnish a ready clue to
the relative amount of run-off in each fall of rain. The measurement of
snowfall is often made merely by determining its depth. For comparison
with rainfall, the rain gauge with receiver and tube withdrawn is used. The
snow which falls is melted, poured into the measuring tube, and measured
in the ordinary way. The U. S. Weather Bureau standard rain gauge, with
measuring stick, may be obtained of H. J. Green, or of J. P. Friez for $5.25.
107. Precipitation records. From the periodic character of precipitation,
rainfall sums, means, and curves have little importance in the careful study
of the habitat. The rainfall curve for the growing season is an aid in
explaining the curve of water-content, and the mean rainfall of a region
gives some idea of its vegetation, though even here the matter of its
distribution is of primary importance. The rain and snow charts published
by the U. S. Weather Bureau furnish data of some importance for the
general study of vegetation, but it is evident that they can play little part in a
system which is founded upon the habitat. Precipitation records, for reasons
of brevity and convenience, are united with wind records, and the form will
be found under the discussion of this factor.
Page 108
WIND
108. Value of readings. On
account of its direct effect upon
humidity, and its consequent
influence upon water-content, the
part which wind plays in a habitat
can not be ignored in a thorough
investigation. It is an important
element in exposure, and
accordingly has a marked
mechanical effect upon the
vegetation of exposed habitats,
alpine slopes, seacoasts, plains, etc.
Fig. 23. Simple anemometer.
Owing to its inconstancy and its
extreme variation in velocity,
single wind readings are absolutely without value. When read in series,
anemometers give some information upon the comparative air movement in
different habitats, but the chance of error is great, except when the breeze is
steady. Anemographs alone give real satisfaction. Accurate results,
however, are not obtainable without a series of two or more in different
habitats, and it is still an open question whether the results obtained justify
the expense. For a completely equipped base station, anemometer,
anemograph, and wind vane are desirable instruments, but the study of the
habitat has by no means reached the stage of precision in which their
general use is necessary.
109. The anemometer in its simplest form is adapted only to readings
made under direct observation, as a sudden change in the direction of the
wind reverses the movement of the indicator needle. This simple wind
gauge, shown in figure 23, has been used for instructional purposes, and to
a slight extent, also, in ascertaining the effect of cover. In constant winds,
successive single readings are found to have value, but, ordinarily, the
observations must be simultaneous. Careful tests of this simple instrument
show that it is essentially accurate. It may be obtained from the C.H.
Stoelting Company, 31 W. Randolph St., Chicago, for $25. The standard
108. Value of readings. On
account of its direct effect upon
humidity, and its consequent
influence upon water-content, the
part which wind plays in a habitat
can not be ignored in a thorough
investigation. It is an important
element in exposure, and
accordingly has a marked
mechanical effect upon the
vegetation of exposed habitats,
alpine slopes, seacoasts, plains, etc.
Fig. 23. Simple anemometer.
Owing to its inconstancy and its
extreme variation in velocity,
single wind readings are absolutely without value. When read in series,
anemometers give some information upon the comparative air movement in
different habitats, but the chance of error is great, except when the breeze is
steady. Anemographs alone give real satisfaction. Accurate results,
however, are not obtainable without a series of two or more in different
habitats, and it is still an open question whether the results obtained justify
the expense. For a completely equipped base station, anemometer,
anemograph, and wind vane are desirable instruments, but the study of the
habitat has by no means reached the stage of precision in which their
general use is necessary.
109. The anemometer in its simplest form is adapted only to readings
made under direct observation, as a sudden change in the direction of the
wind reverses the movement of the indicator needle. This simple wind
gauge, shown in figure 23, has been used for instructional purposes, and to
a slight extent, also, in ascertaining the effect of cover. In constant winds,
successive single readings are found to have value, but, ordinarily, the
observations must be simultaneous. Careful tests of this simple instrument
show that it is essentially accurate. It may be obtained from the C.H.
Stoelting Company, 31 W. Randolph St., Chicago, for $25. The standard
Page 109
anemometer (Fig. 24) is practically
a recording instrument up to 1,000
miles, but as the dials run on
without any indication of the total
number of revolutions, it must be
visited and read each day. This
renders its use difficult for habitats
which are some distance apart.
When exact determinations of
wind values become necessary, the
most successful method is to
establish a series of three standard
anemometers. One of these should
be placed upon the most exposed
part of a typically open habitat, the
second in the most protected part
of the same habitat, while the third Fig. 24. Standard anemometer.
is located in the midst of a
representative forest formation. If
the two habitats are close together, the daily visits can be made without
serious inconvenience. The reading of the registering dials requires detailed
explanation, and for this the reader is referred to the printed directions
which accompany the instrument. In setting up the anemometer it must be
borne in mind that the ecologist desires the wind velocity for a particular
habitat. In consequence, the precautions which the meteorologist takes to
place the instrument at a certain height and well away from surrounding
obstructions do not hold here. Standard anemometers are furnished by H. J.
Green, and J. P. Friez for $25 each.
The anemograph is an anemometer electrically connected with an
automatic register. It is the only instrument adapted to continuous weekly
records in different habitats, but the price, $75 ($25 for the anemometer and
$50 for the register) is practically prohibitive, at least until a complete series
of ecographs for other factors has been obtained.
110. Records. The following form is used as a combined record for
precipitation and wind:
a recording instrument up to 1,000
miles, but as the dials run on
without any indication of the total
number of revolutions, it must be
visited and read each day. This
renders its use difficult for habitats
which are some distance apart.
When exact determinations of
wind values become necessary, the
most successful method is to
establish a series of three standard
anemometers. One of these should
be placed upon the most exposed
part of a typically open habitat, the
second in the most protected part
of the same habitat, while the third Fig. 24. Standard anemometer.
is located in the midst of a
representative forest formation. If
the two habitats are close together, the daily visits can be made without
serious inconvenience. The reading of the registering dials requires detailed
explanation, and for this the reader is referred to the printed directions
which accompany the instrument. In setting up the anemometer it must be
borne in mind that the ecologist desires the wind velocity for a particular
habitat. In consequence, the precautions which the meteorologist takes to
place the instrument at a certain height and well away from surrounding
obstructions do not hold here. Standard anemometers are furnished by H. J.
Green, and J. P. Friez for $25 each.
The anemograph is an anemometer electrically connected with an
automatic register. It is the only instrument adapted to continuous weekly
records in different habitats, but the price, $75 ($25 for the anemometer and
$50 for the register) is practically prohibitive, at least until a complete series
of ecographs for other factors has been obtained.
110. Records. The following form is used as a combined record for
precipitation and wind:
Page 110
RAINFALL WIND
Day Time Formation Station Altitude Exposure Community Base Base
Inches Duration Velocity Heig’t Direction
6:30 Half 2550 N.E. 8
29/8/04 Hiawatha Asterare 1 5 3 ft. N. W.
P.M. gravel m. 17° hours
5:45 10
31/8/04 „ „ „ „ „ Trace 12 „ „
P.M. min.
4:00 2
2/9/04 „ „ „ „ „ .2 7 „ W.
P.M. hours
10:00
3/9/04 „ „ „ „ „ Trace 18 „ „
P.M.
Day Time Formation Station Altitude Exposure Community Base Base
Inches Duration Velocity Heig’t Direction
6:30 Half 2550 N.E. 8
29/8/04 Hiawatha Asterare 1 5 3 ft. N. W.
P.M. gravel m. 17° hours
5:45 10
31/8/04 „ „ „ „ „ Trace 12 „ „
P.M. min.
4:00 2
2/9/04 „ „ „ „ „ .2 7 „ W.
P.M. hours
10:00
3/9/04 „ „ „ „ „ Trace 18 „ „
P.M.
Page 111
SOIL
111. Soil as a factor. In determining the value of the soil as a factor in a
particular habitat, it must be clearly recognized that its importance lies
solely in the control which it exerts upon water-content and nutrient-
content. The former is directly connected with the texture or fineness of the
soil, the latter with its chemical nature. Accordingly, the structure of the soil
and its chemical composition are the fundamental points of attack. These
are not at all of equal value, however. Water is both a food, and a solvent for
the nutrient salts of the soil. Furthermore, the per cent of soluble salts, as
determined in mechanical analyses, is practically the same for all ordinary
soils. Indeed, the variations for the same soil types are as great as for
entirely different types. For these reasons, soluble salt-content may be
ignored except where it is readily seen to be excessive, as in alkaline soils;
and determinations of chemical composition are necessary only in those
soils which contain salts or acids to an injurious degree, e. g., alkaline soils,
peat bogs, humus swamps, etc. The structure of the soil, on the other hand,
in the usual absence of excessive amounts of solutes, absolutely controls the
fate of the water that enters the ground, in addition to its influence upon the
run-off. It determines the amount of gravitation water lost by percolation, as
well as the water that can be raised by capillarity. The resultant of these, the
total soil water or holard, is hence an effect of structure, while the size and
compactness of the particles are conclusive factors in controlling the
chresard. It must be recognized, however, that these are all factors which
enable us to interpret the amount of holard or chresard found in a particular
soil. They have no direct important effect upon the plant, but influence it
only in so far as they affect the water present.
112. The value of soil surveys. The full appreciation of the preeminent
value of water-content, particularly of the chresard, greatly simplifies the
ecological study of soils. The ecologist is primarily concerned with soil
water only in its relation to the plant, and while a fair knowledge of soil
structure is essential to a proper understanding of this, he has little concern
with the detailed study of the problems of soil physics. For the sake of a
proper balance of values, he must avoid the tendency noted elsewhere of
ignoring the claims of the plant, and of studying the soil simply as the seat
111. Soil as a factor. In determining the value of the soil as a factor in a
particular habitat, it must be clearly recognized that its importance lies
solely in the control which it exerts upon water-content and nutrient-
content. The former is directly connected with the texture or fineness of the
soil, the latter with its chemical nature. Accordingly, the structure of the soil
and its chemical composition are the fundamental points of attack. These
are not at all of equal value, however. Water is both a food, and a solvent for
the nutrient salts of the soil. Furthermore, the per cent of soluble salts, as
determined in mechanical analyses, is practically the same for all ordinary
soils. Indeed, the variations for the same soil types are as great as for
entirely different types. For these reasons, soluble salt-content may be
ignored except where it is readily seen to be excessive, as in alkaline soils;
and determinations of chemical composition are necessary only in those
soils which contain salts or acids to an injurious degree, e. g., alkaline soils,
peat bogs, humus swamps, etc. The structure of the soil, on the other hand,
in the usual absence of excessive amounts of solutes, absolutely controls the
fate of the water that enters the ground, in addition to its influence upon the
run-off. It determines the amount of gravitation water lost by percolation, as
well as the water that can be raised by capillarity. The resultant of these, the
total soil water or holard, is hence an effect of structure, while the size and
compactness of the particles are conclusive factors in controlling the
chresard. It must be recognized, however, that these are all factors which
enable us to interpret the amount of holard or chresard found in a particular
soil. They have no direct important effect upon the plant, but influence it
only in so far as they affect the water present.
112. The value of soil surveys. The full appreciation of the preeminent
value of water-content, particularly of the chresard, greatly simplifies the
ecological study of soils. The ecologist is primarily concerned with soil
water only in its relation to the plant, and while a fair knowledge of soil
structure is essential to a proper understanding of this, he has little concern
with the detailed study of the problems of soil physics. For the sake of a
proper balance of values, he must avoid the tendency noted elsewhere of
ignoring the claims of the plant, and of studying the soil simply as the seat
Page 112
of certain physical phenomena. Accordingly, it is felt that mechanical and
chemical analyses, determinations of soluble salt-content, etc., have much
less value than has been commonly supposed. The usual methods of soil
survey, which pay little or no attention to water-content, and none at all to
available water, are practically valueless for ecological research. This
statement does not indicate a failure to appreciate the importance of the
usual soil methods for many agricultural problems, such as the use of
fertilizers, conservation of moisture, etc., though even here to focus the
work upon water-content would give much more fundamental and
serviceable results. For these reasons, slight attention will be paid to
methods of mechanical and chemical analysis. In their stead is given a brief
statement of the origin, structure, and character of soils with especial
reference to water-content.
113. The origin of soils. Rocks form soils in consequence of weathering,
under the influence of physical and biotic factors. Weathering consists of
two processes, disintegration, by which the rock is broken into component
particles of various sizes, and decomposition, in which the rock or its
fragments are resolved into minute particles in consequence of the chemical
disaggregation of its minerals, or of some other chemical change. These
processes are usually concomitant, although, as a rule, one is more evident
than the other. The relation between them is dependent upon the character
of the rock and the forces which act upon it. Hard rocks, i. e., igneous and
metamorphic ones, as a rule disintegrate more rapidly than they decompose;
sedimentary rocks, on the other hand, tend to decompose more rapidly than
they disintegrate. In many cases the two processes go hand in hand. This
difference is the basis for the distinction, first proposed by Thurmann,
between those rocks which weather with difficulty and those which weather
readily. The former were called dysgeogenous, the latter eugeogenous.
Thurmann restricted the application of the first term to those rocks which
produce little soil, but it seems more logical to apply dysgeogenous to those
in which disintegration is markedly in excess of decomposition, and
eugeogenous to those rocks that break down rather readily into fine soils.
With respect to the general character of the soil formed, rocks are
pelogenous, clay-producing, psammogenous, sand-forming, or
pelopsammogenous, producing mixed clay and sand. The first two are
divided into perpelic, hemipelic, oligopelic, perpsammic, etc., with
reference to the readiness with which they are weathered, but this
chemical analyses, determinations of soluble salt-content, etc., have much
less value than has been commonly supposed. The usual methods of soil
survey, which pay little or no attention to water-content, and none at all to
available water, are practically valueless for ecological research. This
statement does not indicate a failure to appreciate the importance of the
usual soil methods for many agricultural problems, such as the use of
fertilizers, conservation of moisture, etc., though even here to focus the
work upon water-content would give much more fundamental and
serviceable results. For these reasons, slight attention will be paid to
methods of mechanical and chemical analysis. In their stead is given a brief
statement of the origin, structure, and character of soils with especial
reference to water-content.
113. The origin of soils. Rocks form soils in consequence of weathering,
under the influence of physical and biotic factors. Weathering consists of
two processes, disintegration, by which the rock is broken into component
particles of various sizes, and decomposition, in which the rock or its
fragments are resolved into minute particles in consequence of the chemical
disaggregation of its minerals, or of some other chemical change. These
processes are usually concomitant, although, as a rule, one is more evident
than the other. The relation between them is dependent upon the character
of the rock and the forces which act upon it. Hard rocks, i. e., igneous and
metamorphic ones, as a rule disintegrate more rapidly than they decompose;
sedimentary rocks, on the other hand, tend to decompose more rapidly than
they disintegrate. In many cases the two processes go hand in hand. This
difference is the basis for the distinction, first proposed by Thurmann,
between those rocks which weather with difficulty and those which weather
readily. The former were called dysgeogenous, the latter eugeogenous.
Thurmann restricted the application of the first term to those rocks which
produce little soil, but it seems more logical to apply dysgeogenous to those
in which disintegration is markedly in excess of decomposition, and
eugeogenous to those rocks that break down rather readily into fine soils.
With respect to the general character of the soil formed, rocks are
pelogenous, clay-producing, psammogenous, sand-forming, or
pelopsammogenous, producing mixed clay and sand. The first two are
divided into perpelic, hemipelic, oligopelic, perpsammic, etc., with
reference to the readiness with which they are weathered, but this
Page 113
distinction is not a very practicable one. The grouping of soils into silicious,
calcareous, argillaceous, etc., with reference to the chemical nature of the
original rock, is of no value to the ecologist, apart from the general clue to
the physical properties which it furnishes.
114. The structure of soils. The water capacity of a soil is a direct result
of the fineness of the particles. Since the water is held as a thin surface film
by each particle or group of them, it follows that the amount of water
increases with the water-holding surface. The latter increases as the
particles become finer and more numerous, and thus produce a greater
aggregate surface. The upward and downward movements of water in the
soil are likewise in immediate connection with the size of particles. The
upward or capillary movement increases as the particles become finer, thus
making the irregular capillary spaces between them smaller, and
magnifying the pull exerted. On the contrary, the downward movement of
gravitation water, i. e., percolation, is retarded by a decrease in the size of
the soil grains and hastened by an increase. Hence, the two properties,
capillarity and porosity, are direct expressions of the structure of the soil, i.
e., of its texture or fineness. Capillarity, however, increases the water-
content of the upper layers permeated by the roots of the plant, while
porosity decreases it. On the basis of these properties alone, soils would fall
into two groups, capillary soils and porous soils, the former fine-grained
and of high water-content, the latter coarse-grained and with relatively little
water. A third factor, however, of great importance must be taken into
account. This is the pull exerted upon each water film by the soil particle
itself. This pull apparently increases in strength as the film grows thinner,
and explains why it finally becomes impossible for the root-hairs to draw
moisture from the soil. This property, like capillarity, is most pronounced in
fine-grained soils, such as clays, and is least evident in the coarser sands
and gravels. It seems to furnish the direct explanation of non-available
water, and, in consequence, to indicate that the chresard is an immediate
result of soil texture.
115. Mechanical analysis. From the above it is evident that, with the
same rainfall, coarse soils will be relatively dry, and fine soils
correspondingly moist. However, this difference in holard is somewhat
counterbalanced by the fact that the chresard is much greater in the former
than in the latter. The basis of these relations can be obtained only from a
study of the texture of the soil. The usual method of doing this is by
calcareous, argillaceous, etc., with reference to the chemical nature of the
original rock, is of no value to the ecologist, apart from the general clue to
the physical properties which it furnishes.
114. The structure of soils. The water capacity of a soil is a direct result
of the fineness of the particles. Since the water is held as a thin surface film
by each particle or group of them, it follows that the amount of water
increases with the water-holding surface. The latter increases as the
particles become finer and more numerous, and thus produce a greater
aggregate surface. The upward and downward movements of water in the
soil are likewise in immediate connection with the size of particles. The
upward or capillary movement increases as the particles become finer, thus
making the irregular capillary spaces between them smaller, and
magnifying the pull exerted. On the contrary, the downward movement of
gravitation water, i. e., percolation, is retarded by a decrease in the size of
the soil grains and hastened by an increase. Hence, the two properties,
capillarity and porosity, are direct expressions of the structure of the soil, i.
e., of its texture or fineness. Capillarity, however, increases the water-
content of the upper layers permeated by the roots of the plant, while
porosity decreases it. On the basis of these properties alone, soils would fall
into two groups, capillary soils and porous soils, the former fine-grained
and of high water-content, the latter coarse-grained and with relatively little
water. A third factor, however, of great importance must be taken into
account. This is the pull exerted upon each water film by the soil particle
itself. This pull apparently increases in strength as the film grows thinner,
and explains why it finally becomes impossible for the root-hairs to draw
moisture from the soil. This property, like capillarity, is most pronounced in
fine-grained soils, such as clays, and is least evident in the coarser sands
and gravels. It seems to furnish the direct explanation of non-available
water, and, in consequence, to indicate that the chresard is an immediate
result of soil texture.
115. Mechanical analysis. From the above it is evident that, with the
same rainfall, coarse soils will be relatively dry, and fine soils
correspondingly moist. However, this difference in holard is somewhat
counterbalanced by the fact that the chresard is much greater in the former
than in the latter. The basis of these relations can be obtained only from a
study of the texture of the soil. The usual method of doing this is by
Page 114
mechanical analysis. This is far
from satisfactory, since the use of
the sieves often brings about the
disaggregation of groups of
particles which act as units in the
soil. Furthermore, the analysis
affords no exact evidence of the
compactness of the soil in nature,
and tests of capillarity and porosity
made with soil samples out of
position are open to serious error.
Nevertheless, mechanical analyses
furnish results of some value by
making it possible to compare soils
upon the basis of texture. For
ecological purposes, minute
analyses are undesirable; their
value in any work is doubtful. A
Fig. 25. Sieves for soil analysis.
separation of soil into gravel, sand,
and silt-clay is sufficient, since the
relative proportion of these will explain the holard and chresard of the soil
concerned. The latter are also affected in rich soils, especially of forests, by
the organic matter present. If this is in a finely divided condition, the
amount is determined by calcining. When a definite layer of leafmold is
present, as in forests and thickets, its water-value is found separately, since
its power of retaining water is altogether out of proportion to its weight.
116. Kinds of soils. It is very doubtful whether it is worth while to
attempt to distinguish soils upon the basis of mechanical analysis.
Unquestionably, the most satisfactory method is to distinguish them with
respect to holard and chresard, and to regard texture as of secondary
importance. A series of soil classes which comprise various soil types has
been proposed by the U. S. Bureau[7] of Soils as follows: (1) stony loam, (2)
gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine sand, (7) sandy
loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) silt loam, (12)
clay loam, (13) clay, (14) adobe. These are based entirely upon mechanical
analyses, and in some cases are too closely related to be useful. The line
between them can nowhere be sharply drawn. Indeed, the variation within
from satisfactory, since the use of
the sieves often brings about the
disaggregation of groups of
particles which act as units in the
soil. Furthermore, the analysis
affords no exact evidence of the
compactness of the soil in nature,
and tests of capillarity and porosity
made with soil samples out of
position are open to serious error.
Nevertheless, mechanical analyses
furnish results of some value by
making it possible to compare soils
upon the basis of texture. For
ecological purposes, minute
analyses are undesirable; their
value in any work is doubtful. A
Fig. 25. Sieves for soil analysis.
separation of soil into gravel, sand,
and silt-clay is sufficient, since the
relative proportion of these will explain the holard and chresard of the soil
concerned. The latter are also affected in rich soils, especially of forests, by
the organic matter present. If this is in a finely divided condition, the
amount is determined by calcining. When a definite layer of leafmold is
present, as in forests and thickets, its water-value is found separately, since
its power of retaining water is altogether out of proportion to its weight.
116. Kinds of soils. It is very doubtful whether it is worth while to
attempt to distinguish soils upon the basis of mechanical analysis.
Unquestionably, the most satisfactory method is to distinguish them with
respect to holard and chresard, and to regard texture as of secondary
importance. A series of soil classes which comprise various soil types has
been proposed by the U. S. Bureau[7] of Soils as follows: (1) stony loam, (2)
gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine sand, (7) sandy
loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) silt loam, (12)
clay loam, (13) clay, (14) adobe. These are based entirely upon mechanical
analyses, and in some cases are too closely related to be useful. The line
between them can nowhere be sharply drawn. Indeed, the variation within
Page 115
one class is so great that soils have frequently been referred to the wrong
group. Thus, Cassadaga sand (gravel 22 per cent, sand 43 per cent, silt 21
per cent, clay 10 per cent) is more closely related to Oxnard sandy loam
(26–37–18–12) and to Afton fine sandy loam (28–43–18–8) than to Coral
sand (61–29–3–4), Galveston sand (6–91–1–1), or Salt Lake sand (84–15–
1–0). Elsinore sandy loam (8–38–35–10) is much nearer to Hanford fine
sandy loam (9–36–33–14) than to Billings sandy loam (1–60–22–11) or to
Utuado sandy loam (48–23–19–8). The soil types are much more confused,
and for ecological purposes at least are entirely valueless. Lake Charles fine
sandy loam has the composition, 1–34–52–9; Vernon fine sandy loam, 1–
37–54–7, while many other so-called types show nearly the same degree of
identity.
117. The chemical nature of soils. The effect of alkaline and acid
substances in the soil upon water-content and the activities of the plant is
far from being well understood. It is generally recognized that salts and
acids tend to inhibit the absorptive power of the root-hairs. In the case of
saline soils, this inhibitive effect seems to be established, but the action of
acids in bogs and swamps is still an open question. It is probable that the
influence of organic acid has been overestimated, and that the curious
anomaly of a structural xerophyte in a swamp is to be explained by the
stability of the ancestral type and by the law of extremes. Apart from the
effect which excessive amounts of acids and salts may have in reducing the
chresard, the chemical character of the soil is powerless to produce
structural modification in the plant. Since Thurmann’s researches there has
been no real support of the contention that the chemical properties of the
soil, not its physical nature, are the decisive factors in the distribution and
adaptation of plants. It is not sufficient that the vegetation of a silicious soil
differs from that of a calcareous one. A soil can modify the plants upon it
only though its water-content, or the solutes it contains. Hence, the
chemical composition of the original rock is immaterial, except in so far as
it modifies these two factors. Humus, moreover, while an important factor
in growth, has no formative influence beyond that which it exerts through
water-content.
group. Thus, Cassadaga sand (gravel 22 per cent, sand 43 per cent, silt 21
per cent, clay 10 per cent) is more closely related to Oxnard sandy loam
(26–37–18–12) and to Afton fine sandy loam (28–43–18–8) than to Coral
sand (61–29–3–4), Galveston sand (6–91–1–1), or Salt Lake sand (84–15–
1–0). Elsinore sandy loam (8–38–35–10) is much nearer to Hanford fine
sandy loam (9–36–33–14) than to Billings sandy loam (1–60–22–11) or to
Utuado sandy loam (48–23–19–8). The soil types are much more confused,
and for ecological purposes at least are entirely valueless. Lake Charles fine
sandy loam has the composition, 1–34–52–9; Vernon fine sandy loam, 1–
37–54–7, while many other so-called types show nearly the same degree of
identity.
117. The chemical nature of soils. The effect of alkaline and acid
substances in the soil upon water-content and the activities of the plant is
far from being well understood. It is generally recognized that salts and
acids tend to inhibit the absorptive power of the root-hairs. In the case of
saline soils, this inhibitive effect seems to be established, but the action of
acids in bogs and swamps is still an open question. It is probable that the
influence of organic acid has been overestimated, and that the curious
anomaly of a structural xerophyte in a swamp is to be explained by the
stability of the ancestral type and by the law of extremes. Apart from the
effect which excessive amounts of acids and salts may have in reducing the
chresard, the chemical character of the soil is powerless to produce
structural modification in the plant. Since Thurmann’s researches there has
been no real support of the contention that the chemical properties of the
soil, not its physical nature, are the decisive factors in the distribution and
adaptation of plants. It is not sufficient that the vegetation of a silicious soil
differs from that of a calcareous one. A soil can modify the plants upon it
only though its water-content, or the solutes it contains. Hence, the
chemical composition of the original rock is immaterial, except in so far as
it modifies these two factors. Humus, moreover, while an important factor
in growth, has no formative influence beyond that which it exerts through
water-content.
Page 116
PHYSIOGRAPHY
118. Factors. The physiographic factors of a definite habitat are altitude,
exposure, slope, and surface. In addition, topography is a general though
less tangible factor of regions, while the dynamic forces of weathering,
erosion and sedimentation play a fundamental role in the change of habitats.
It is evident, however, that these, except where they affect the destruction of
vegetation directly, can operate upon the plant only through more direct
factors, such as water, light, and temperature. While they are themselves not
susceptible of measurement, they can often be expressed in terms of
determinable factors, i. e., slope, exposure, and surface. Fundamentally,
they constitute the forces which change one habitat into another, and, in
consequence, are really to be considered as the factors which produce
succession. The static features of physiography, altitude, etc., lend
themselves readily to determination by means of precise instruments. These
factors, though by no means negligible, are remote, and consequently their
mere measurement is insufficient to indicate the nature or extent of their
influence upon the plant. It is necessary to determine also the manner and
degree in which they affect other factors, a task yet to be done. Readings of
altitude, slope, and exposure are so easily made that the student must
carefully avoid the tendency to let them stand at their own value, which is
slight. Instead, they should be made the starting point for ascertaining the
differences which they produce in water-content, humidity wind, and
temperature.
Altitude
119. Analysis into factors. Of all physiographic features, altitude is the
most difficult to resolve into simple factors. Because of general geographic
relations, it has a certain connection with rainfall, but this is vague and
inconstant. Obviously, in its influence upon the plant, altitude is really
pressure, and in consequence its effect is exerted upon the climatic and not
the edaphic factors of the habitat. Theoretically, the decrease of air pressure
in the increased altitude directly affects humidity, light, and temperature.
118. Factors. The physiographic factors of a definite habitat are altitude,
exposure, slope, and surface. In addition, topography is a general though
less tangible factor of regions, while the dynamic forces of weathering,
erosion and sedimentation play a fundamental role in the change of habitats.
It is evident, however, that these, except where they affect the destruction of
vegetation directly, can operate upon the plant only through more direct
factors, such as water, light, and temperature. While they are themselves not
susceptible of measurement, they can often be expressed in terms of
determinable factors, i. e., slope, exposure, and surface. Fundamentally,
they constitute the forces which change one habitat into another, and, in
consequence, are really to be considered as the factors which produce
succession. The static features of physiography, altitude, etc., lend
themselves readily to determination by means of precise instruments. These
factors, though by no means negligible, are remote, and consequently their
mere measurement is insufficient to indicate the nature or extent of their
influence upon the plant. It is necessary to determine also the manner and
degree in which they affect other factors, a task yet to be done. Readings of
altitude, slope, and exposure are so easily made that the student must
carefully avoid the tendency to let them stand at their own value, which is
slight. Instead, they should be made the starting point for ascertaining the
differences which they produce in water-content, humidity wind, and
temperature.
Altitude
119. Analysis into factors. Of all physiographic features, altitude is the
most difficult to resolve into simple factors. Because of general geographic
relations, it has a certain connection with rainfall, but this is vague and
inconstant. Obviously, in its influence upon the plant, altitude is really
pressure, and in consequence its effect is exerted upon the climatic and not
the edaphic factors of the habitat. Theoretically, the decrease of air pressure
in the increased altitude directly affects humidity, light, and temperature.
Page 117
Actually, while there is unquestionably a decrease in the absorption of the
light and heat rays owing to the fact that they traverse less atmosphere,
which is at the same time less dense, this seems to be negligible.
Photometric readings at elevations of 6,000 and 14,000 feet have so far
failed to show more than slight differences, which are altogether too small
to be efficient. The effect upon humidity is greater, but the degree is
uncertain. Continuous psychrographic records at different elevations for a
full season, at least, will be necessary to determine this, since the
psychrometric readings so far made, while referred to a base psychrograph,
are too scattered to be conclusive. Finally, the length of the season, itself a
composite, is directly dependent upon the altitude. This relation, though
obscure, rests chiefly upon the rarefaction of the air which prevents the
accumulation of heat in both the soil and the air.
120. The barometer. To secure
convenience and accuracy in the
determination of altitude, it is
necessary to use both a mercurial
and an aneroid barometer. The
latter is by far the most serviceable
for field work, but it requires
frequent standardizing by means of
the former. The mercurial form is
much more accurate and should be
read daily in the base station. It is
practically impossible to carry it in
the field, except in the so-called
mountain form, which is of great
service in establishing the altitudes
of a series of stations. In use the
aneroid barometer may be checked
daily by the mercurial standard, or
it may be set at the altitude of the
Fig. 26. Aneroid barometer. base station, thus giving a direct
reading. After the normal pressure
at the base has once been ascertained, however, the most satisfactory
method is to set the aneroid each day by the standard, at the same time
noting the pressure deviation in feet of elevation (see p. 46). The absolute
light and heat rays owing to the fact that they traverse less atmosphere,
which is at the same time less dense, this seems to be negligible.
Photometric readings at elevations of 6,000 and 14,000 feet have so far
failed to show more than slight differences, which are altogether too small
to be efficient. The effect upon humidity is greater, but the degree is
uncertain. Continuous psychrographic records at different elevations for a
full season, at least, will be necessary to determine this, since the
psychrometric readings so far made, while referred to a base psychrograph,
are too scattered to be conclusive. Finally, the length of the season, itself a
composite, is directly dependent upon the altitude. This relation, though
obscure, rests chiefly upon the rarefaction of the air which prevents the
accumulation of heat in both the soil and the air.
120. The barometer. To secure
convenience and accuracy in the
determination of altitude, it is
necessary to use both a mercurial
and an aneroid barometer. The
latter is by far the most serviceable
for field work, but it requires
frequent standardizing by means of
the former. The mercurial form is
much more accurate and should be
read daily in the base station. It is
practically impossible to carry it in
the field, except in the so-called
mountain form, which is of great
service in establishing the altitudes
of a series of stations. In use the
aneroid barometer may be checked
daily by the mercurial standard, or
it may be set at the altitude of the
Fig. 26. Aneroid barometer. base station, thus giving a direct
reading. After the normal pressure
at the base has once been ascertained, however, the most satisfactory
method is to set the aneroid each day by the standard, at the same time
noting the pressure deviation in feet of elevation (see p. 46). The absolute
Page 118
elevation of the various stations of a series may be determined either by
adding or subtracting this deviation from the actual reading at the station, or
by noting the change from the base station, and then adding or subtracting
this from the normal of the latter. When it is impossible to check the aneroid
by means of a mercurial barometer, the average of a series of readings made
at different days at one station, especially if taken during settled weather,
will practically eliminate the daily fluctuations, and yield a result essentially
accurate. Even in this event, the accuracy of the aneroid should be checked
as often as possible, since the mechanism may go wrong at any time. The
barograph, while a valuable instrument for base stations, is not at all
necessary. These instruments can be obtained from all makers of
meteorological apparatus, such as H. J. Green, and J. P. Friez. Aneroid
barometers reading to 16,000 feet cost about $20; the price of the Richards
aneroid barograph is $45. Ordinary observatory barometers cost $30–$40;
the standard instrument sells at $75–$100. The mountain barometer, which
is altogether the most serviceable for the ecologist, ranges from $30–$55,
depending upon accessories, etc.
Slope
121. Concept. This term is used in the ordinary sense to indicate the
relation of the surface of a habitat to the horizon. Although it is a complex
of factors, or rather influences several factors, these are readily
determinable. The primary effect of slope is seen in the control of run-off
and drainage, and consequently of water-content, although these are
likewise affected by soil texture and by surface. Slope, moreover, as a
concomitant of exposure, has an important bearing upon light and heat by
virtue of determining the angle of incidence, and also upon wind, and,
through it, upon the distribution of snow. At present, while it can be
expressed definitely in degrees, it has not yet been connected quantitatively
with more direct factors. This is, however, not a difficult task, and it is
probable that we shall soon come to express slope principally in amount of
run-off, and of incident heat.
122. The clinometer. In the simplest form, this instrument is merely a
semicircle of paper, with each half graduated from 1–90°. It is mounted on
a board and placed base upward, upon a wooden strip, 2 feet long and 2
adding or subtracting this deviation from the actual reading at the station, or
by noting the change from the base station, and then adding or subtracting
this from the normal of the latter. When it is impossible to check the aneroid
by means of a mercurial barometer, the average of a series of readings made
at different days at one station, especially if taken during settled weather,
will practically eliminate the daily fluctuations, and yield a result essentially
accurate. Even in this event, the accuracy of the aneroid should be checked
as often as possible, since the mechanism may go wrong at any time. The
barograph, while a valuable instrument for base stations, is not at all
necessary. These instruments can be obtained from all makers of
meteorological apparatus, such as H. J. Green, and J. P. Friez. Aneroid
barometers reading to 16,000 feet cost about $20; the price of the Richards
aneroid barograph is $45. Ordinary observatory barometers cost $30–$40;
the standard instrument sells at $75–$100. The mountain barometer, which
is altogether the most serviceable for the ecologist, ranges from $30–$55,
depending upon accessories, etc.
Slope
121. Concept. This term is used in the ordinary sense to indicate the
relation of the surface of a habitat to the horizon. Although it is a complex
of factors, or rather influences several factors, these are readily
determinable. The primary effect of slope is seen in the control of run-off
and drainage, and consequently of water-content, although these are
likewise affected by soil texture and by surface. Slope, moreover, as a
concomitant of exposure, has an important bearing upon light and heat by
virtue of determining the angle of incidence, and also upon wind, and,
through it, upon the distribution of snow. At present, while it can be
expressed definitely in degrees, it has not yet been connected quantitatively
with more direct factors. This is, however, not a difficult task, and it is
probable that we shall soon come to express slope principally in amount of
run-off, and of incident heat.
122. The clinometer. In the simplest form, this instrument is merely a
semicircle of paper, with each half graduated from 1–90°. It is mounted on
a board and placed base upward, upon a wooden strip, 2 feet long and 2
Page 119
inches wide, which has a true edge.
At the center of the circle is
attached a line and plummet for
reading the perpendicular. A more
convenient form is shown in figure
28, which is both clinometer and
compass. This also necessitates the
use of a basing strip to eliminate
the inequalities of the surface. The
dial face is graduated to show
inches of rise per yard, as well as
the number of degrees, but the
latter, as the simpler term, is
preferable for ecological work. In
making a reading, the basing strip
is placed upon a representative area
of the slope, and pressed down
firmly to equalize slight
irregularities. The clinometer is
moved slightly along the upper
edge, causing the marker to swing
freely. After the latter comes to
rest, the instrument is carefully
turned upon its back, when the
angle of the slope in degrees may
be read directly. Two or three such
readings in different areas will
suffice for the entire habitat, unless
it be extremely irregular. The
clinometer with compass may be
obtained from the Keuffel and
Esser Company, 111 Madison St., Fig. 27. Mountain barometer: (a) in
Chicago, Illinois, for $5. carrying case; (b) set up for use.
At the center of the circle is
attached a line and plummet for
reading the perpendicular. A more
convenient form is shown in figure
28, which is both clinometer and
compass. This also necessitates the
use of a basing strip to eliminate
the inequalities of the surface. The
dial face is graduated to show
inches of rise per yard, as well as
the number of degrees, but the
latter, as the simpler term, is
preferable for ecological work. In
making a reading, the basing strip
is placed upon a representative area
of the slope, and pressed down
firmly to equalize slight
irregularities. The clinometer is
moved slightly along the upper
edge, causing the marker to swing
freely. After the latter comes to
rest, the instrument is carefully
turned upon its back, when the
angle of the slope in degrees may
be read directly. Two or three such
readings in different areas will
suffice for the entire habitat, unless
it be extremely irregular. The
clinometer with compass may be
obtained from the Keuffel and
Esser Company, 111 Madison St., Fig. 27. Mountain barometer: (a) in
Chicago, Illinois, for $5. carrying case; (b) set up for use.
Page 120
Fig. 28. Combined clinometer and compass.
123. The trechometer. For measuring the effect of slope upon run-off, a
simple instrument called the trechometer (τρέχω, to run off) has been
devised. This consists merely of a metal tank, 3 × 4 × 12 inches, holding
144 cubic inches of water, with an opening ¼ × 12 inches at the base in
front, closed by a tight-fitting slide. Three metal strips, 2 × 12 inches, are
fastened to the front of the tank in such a way as to enclose a square foot of
soil into which the strips penetrate an inch. In the front strip is an opening, 1
inch square, provided with a drip from which the run-off is collected in a
measuring vessel. In use, the instrument is put in position with the metal
rim forced down 1 inch into the soil; the tank is filled, the graduate put in
place, and the slide raised. The run-off for a square foot is the amount of
water caught by the graduate, and is represented in cubic inches per square
foot. For obtaining results which express slope alone, comparisons must be
made upon the same soil, from which all cover, dead and living, has been
removed. They must be as closely together in time as possible, at least
during the same day, as rain or evaporation will cause considerable error. It
is obvious that with the same slope or on a level the trechometer may also
be used to advantage to determine the absorptive power of soils of different
texture. It serves well a similar purpose when used in different habitats to
measure the composite action of slope, soil, and cover in dividing the
rainfall into run-off and absorbed water.
123. The trechometer. For measuring the effect of slope upon run-off, a
simple instrument called the trechometer (τρέχω, to run off) has been
devised. This consists merely of a metal tank, 3 × 4 × 12 inches, holding
144 cubic inches of water, with an opening ¼ × 12 inches at the base in
front, closed by a tight-fitting slide. Three metal strips, 2 × 12 inches, are
fastened to the front of the tank in such a way as to enclose a square foot of
soil into which the strips penetrate an inch. In the front strip is an opening, 1
inch square, provided with a drip from which the run-off is collected in a
measuring vessel. In use, the instrument is put in position with the metal
rim forced down 1 inch into the soil; the tank is filled, the graduate put in
place, and the slide raised. The run-off for a square foot is the amount of
water caught by the graduate, and is represented in cubic inches per square
foot. For obtaining results which express slope alone, comparisons must be
made upon the same soil, from which all cover, dead and living, has been
removed. They must be as closely together in time as possible, at least
during the same day, as rain or evaporation will cause considerable error. It
is obvious that with the same slope or on a level the trechometer may also
be used to advantage to determine the absorptive power of soils of different
texture. It serves well a similar purpose when used in different habitats to
measure the composite action of slope, soil, and cover in dividing the
rainfall into run-off and absorbed water.
Page 121
Exposure
124. Exposure refers primarily to the direction toward which a slope
faces, i. e., its exposition or insolation with respect to sun and wind. It is not
altogether separable from slope, however, inasmuch as the angle of the
slope has some effect upon the degree of exposure. The chief influence of
exposure is exerted through temperature, since slopes longest exposed to
the sun’s rays receive the most heat. This is supplemented in an important
degree by the fact that a group of rays 1 foot square will occupy this area
only on slopes upon which they fall at right angles. In all other cases the
rays are spread over a longer area, with a consequent reduction in the
amount of heat received. This effect is felt principally in evaporation from
the soil, and in soil temperatures. For the leaf, it is largely if not entirely
negligible, since the angle of incidence is determined by the position of the
leaf, which is the same for each species whether on the level or upon a
slope. On this account, exposure has little or no bearing upon light, except
that the total amount of light received by the aggregate vegetation of a slope
will be greater than for a level area of the same size. The effect of wind
varies with the exposure. It is naturally most pronounced in those directions
from which the prevailing dry or cold winds blow, and it is greatly
emphasized by the fact that the opposite exposure is correspondingly
protected. The influence of wind, especially in producing evaporation from
the plant and the soil, increases with the slope, since the mutual protection
of the plants, or that afforded the soil by the cover, is much reduced. Finally,
the distribution of the snow by the wind, a matter of considerable
importance for early spring vegetation, is largely determined by exposure.
Exposure is expressed directly in terms of direction, to which is added
the angle of the slope. A good field compass, reading to twelve points, is
sufficient. It should be checked, of course, by the declination of the needle
at the place under observation. A convenient instrument is the one already
mentioned, in which compass and clinometer are combined, since these are
regularly used at the same time.
125. Surface. The most important consideration with respect to surface is
the presence or absence of cover, and the character of the latter. With the
exception of snow, cover is, however, a question of vegetation, living and
dead, and consequently is to be referred to the discussion of biotic factors.
124. Exposure refers primarily to the direction toward which a slope
faces, i. e., its exposition or insolation with respect to sun and wind. It is not
altogether separable from slope, however, inasmuch as the angle of the
slope has some effect upon the degree of exposure. The chief influence of
exposure is exerted through temperature, since slopes longest exposed to
the sun’s rays receive the most heat. This is supplemented in an important
degree by the fact that a group of rays 1 foot square will occupy this area
only on slopes upon which they fall at right angles. In all other cases the
rays are spread over a longer area, with a consequent reduction in the
amount of heat received. This effect is felt principally in evaporation from
the soil, and in soil temperatures. For the leaf, it is largely if not entirely
negligible, since the angle of incidence is determined by the position of the
leaf, which is the same for each species whether on the level or upon a
slope. On this account, exposure has little or no bearing upon light, except
that the total amount of light received by the aggregate vegetation of a slope
will be greater than for a level area of the same size. The effect of wind
varies with the exposure. It is naturally most pronounced in those directions
from which the prevailing dry or cold winds blow, and it is greatly
emphasized by the fact that the opposite exposure is correspondingly
protected. The influence of wind, especially in producing evaporation from
the plant and the soil, increases with the slope, since the mutual protection
of the plants, or that afforded the soil by the cover, is much reduced. Finally,
the distribution of the snow by the wind, a matter of considerable
importance for early spring vegetation, is largely determined by exposure.
Exposure is expressed directly in terms of direction, to which is added
the angle of the slope. A good field compass, reading to twelve points, is
sufficient. It should be checked, of course, by the declination of the needle
at the place under observation. A convenient instrument is the one already
mentioned, in which compass and clinometer are combined, since these are
regularly used at the same time.
125. Surface. The most important consideration with respect to surface is
the presence or absence of cover, and the character of the latter. With the
exception of snow, cover is, however, a question of vegetation, living and
dead, and consequently is to be referred to the discussion of biotic factors.
Page 122
The surface of the soil itself often shows irregularities which must be taken
into account. Such are the rocks of boulder and rock fields, the hummocks
of meadows and bogs, the mounds of prairie dog towns, the innumerable
minute gullies and ridges of bad lands, the raised tufts of sand-hills, etc. The
influence of these is not profound, but they do have an appreciable effect
upon the run-off, temperature, and wind. In many cases, this is distinctly
measurable, but as a rule little more can be done than to indicate that the
surface is even or uneven, and to describe the degree and kind of
unevenness.
126. Record of physiographic factors. Altitude, slope, exposure, and
surface are essentially constant factors, and are determined once for all,
after a few check readings have been made, except in those relatively rare
habitats in which dynamic forces are very active. The form of record used is
the following:
into account. Such are the rocks of boulder and rock fields, the hummocks
of meadows and bogs, the mounds of prairie dog towns, the innumerable
minute gullies and ridges of bad lands, the raised tufts of sand-hills, etc. The
influence of these is not profound, but they do have an appreciable effect
upon the run-off, temperature, and wind. In many cases, this is distinctly
measurable, but as a rule little more can be done than to indicate that the
surface is even or uneven, and to describe the degree and kind of
unevenness.
126. Record of physiographic factors. Altitude, slope, exposure, and
surface are essentially constant factors, and are determined once for all,
after a few check readings have been made, except in those relatively rare
habitats in which dynamic forces are very active. The form of record used is
the following:
Page 123
DATE FORMATION STATION GROUP ALTITUDE SLOPE EXPOSURE SURFACE
10/7/02 Gravel slide Golf Links Eriogonare 2700 m. 23° N.N.W. Even
„ Brook bank Jack Brook Violare 2550 m. 5° E.N.E. „
„ Half gravel Hiawatha Achilleare 2600 m. 14° E. Uneven
„ Spruce Milky Way Opulasterare 2625 m. 12° N. Even
127. Topography. As heretofore indicated, questions pertaining to the
form and development of the land concern groups of habitats within which
each habitat is the unit of investigation after the manner already laid down.
A knowledge of topography is essential to the accurate mapping of a region,
for which the simple methods of plane table and contour work are
employed, while the geology of the surface is of primary importance in the
study of successions.
10/7/02 Gravel slide Golf Links Eriogonare 2700 m. 23° N.N.W. Even
„ Brook bank Jack Brook Violare 2550 m. 5° E.N.E. „
„ Half gravel Hiawatha Achilleare 2600 m. 14° E. Uneven
„ Spruce Milky Way Opulasterare 2625 m. 12° N. Even
127. Topography. As heretofore indicated, questions pertaining to the
form and development of the land concern groups of habitats within which
each habitat is the unit of investigation after the manner already laid down.
A knowledge of topography is essential to the accurate mapping of a region,
for which the simple methods of plane table and contour work are
employed, while the geology of the surface is of primary importance in the
study of successions.
Page 124
BIOTIC FACTORS
128. Influence and importance. Biotic factors are animals and plants.
With respect to influence they are usually remote, rarely direct.
Nevertheless, they often play a decisive part in the vegetation. Their effect
is, as a rule, felt directly by the formation rather than the habitat, but in
either case the one reacts upon the other. Such factors are not themselves
susceptible of exact measurement, but their influence upon the habitat is
usually measurable in terms of the physical factors affected. In the case of
biotic factors, it must be distinctly understood that these are not properly
factors of the habitat as a physical complex, but that they are rather to be
considered as reactions exerted by the effect, or formation, upon the cause
or habitat. This is most especially true of plants.
129. Animals. The activities of man fall into two classes: (1) those that
destroy vegetation, and (2) those that modify it. There are rare instances
also where the work of man has changed a new or already denuded habitat.
In the cases where the vegetation is destroyed, the habitat itself is
sufficiently changed to permit the effect to be measured by physical factor
instruments. Otherwise, the influence is felt only by the formation, as when
man makes possible the migration of weeds, and it can be measured in
terms of invasion by the quadrat alone. It becomes especially evident, then,
in the case of man’s activities, that where they produce a denuded habitat
they are to be regarded as factors in the habitat; when they merely affect the
formation, this is not strictly true. The changes wrought by other animals
are essentially the same as those produced by man. They are not so marked
nor so important, but their relation to habitat and formation is the same. As
a rule, however, they affect the habitat much less than they do the
formation.
130. Plants. As a dead cover, vegetation is a factor of the habitat proper,
but it has relatively little importance, since it occurs regularly during the
resting period. Its chief effects are in modifying soil temperature, and in
holding snow and rain, and thereby increasing the water-content. By its
gradual decay, moreover, it not only adds humus to the soil, but it thereby
increases the water-retaining capacity of the latter also. The cover of living
vegetation reacts upon the habitat in a much more vital fashion, exerting a
128. Influence and importance. Biotic factors are animals and plants.
With respect to influence they are usually remote, rarely direct.
Nevertheless, they often play a decisive part in the vegetation. Their effect
is, as a rule, felt directly by the formation rather than the habitat, but in
either case the one reacts upon the other. Such factors are not themselves
susceptible of exact measurement, but their influence upon the habitat is
usually measurable in terms of the physical factors affected. In the case of
biotic factors, it must be distinctly understood that these are not properly
factors of the habitat as a physical complex, but that they are rather to be
considered as reactions exerted by the effect, or formation, upon the cause
or habitat. This is most especially true of plants.
129. Animals. The activities of man fall into two classes: (1) those that
destroy vegetation, and (2) those that modify it. There are rare instances
also where the work of man has changed a new or already denuded habitat.
In the cases where the vegetation is destroyed, the habitat itself is
sufficiently changed to permit the effect to be measured by physical factor
instruments. Otherwise, the influence is felt only by the formation, as when
man makes possible the migration of weeds, and it can be measured in
terms of invasion by the quadrat alone. It becomes especially evident, then,
in the case of man’s activities, that where they produce a denuded habitat
they are to be regarded as factors in the habitat; when they merely affect the
formation, this is not strictly true. The changes wrought by other animals
are essentially the same as those produced by man. They are not so marked
nor so important, but their relation to habitat and formation is the same. As
a rule, however, they affect the habitat much less than they do the
formation.
130. Plants. As a dead cover, vegetation is a factor of the habitat proper,
but it has relatively little importance, since it occurs regularly during the
resting period. Its chief effects are in modifying soil temperature, and in
holding snow and rain, and thereby increasing the water-content. By its
gradual decay, moreover, it not only adds humus to the soil, but it thereby
increases the water-retaining capacity of the latter also. The cover of living
vegetation reacts upon the habitat in a much more vital fashion, exerting a
Page 125
powerful effect upon every physical factor of the habitat. The factors thus
affected are distinctly measurable though it is often impossible to determine
just how much of the factor is directly traceable to the vegetation. This is a
simple problem in the case of most aerial factors, especially light, but it is
extremely difficult for soil factors, such as water-content and soil texture. In
the case of all habitats covered with formations, by far the great majority, it
is impossible as well as unnecessary to separate the physical factors of the
habitat proper from the reaction upon them which the plant covering exerts.
Indeed, the great differentiation of habitats is largely due to the universal
principle that in vegetation the effect or formation always reacts upon the
cause or habitat in such a way as to modify it. As fundamental causes of
succession, the discussion of the various reactions of vegetation is reserved
for another place.
Methods of Habitat Investigation
131. The use of the various instruments previously described depends
largely upon the preponderance of simple instruments or recording ones.
The former necessitate a number of well-trained assistants; the latter require
only a part of the time of one investigator. For the most satisfactory results,
however, an assistant is all but indispensable. Since simple instruments are
most easily obtained because of their cheapness, and are especially adapted
to purposes of instruction, the method of using them will be described first,
and then that of ecograph batteries.
affected are distinctly measurable though it is often impossible to determine
just how much of the factor is directly traceable to the vegetation. This is a
simple problem in the case of most aerial factors, especially light, but it is
extremely difficult for soil factors, such as water-content and soil texture. In
the case of all habitats covered with formations, by far the great majority, it
is impossible as well as unnecessary to separate the physical factors of the
habitat proper from the reaction upon them which the plant covering exerts.
Indeed, the great differentiation of habitats is largely due to the universal
principle that in vegetation the effect or formation always reacts upon the
cause or habitat in such a way as to modify it. As fundamental causes of
succession, the discussion of the various reactions of vegetation is reserved
for another place.
Methods of Habitat Investigation
131. The use of the various instruments previously described depends
largely upon the preponderance of simple instruments or recording ones.
The former necessitate a number of well-trained assistants; the latter require
only a part of the time of one investigator. For the most satisfactory results,
however, an assistant is all but indispensable. Since simple instruments are
most easily obtained because of their cheapness, and are especially adapted
to purposes of instruction, the method of using them will be described first,
and then that of ecograph batteries.
Page 126
THE METHOD OF SIMPLE INSTRUMENTS
Fig. 29. Series of stations: I, at Minnehaha; II, at Lincoln in the prairie formation.
132. Choice of stations. This method is based upon simultaneous
readings by means of simple instruments in a series of habitats, or of
stations in a single habitat. Such readings are necessary for the variable
atmospheric factors, humidity, light, temperature, and wind. Frequent
readings suffice for water-content and precipitation, while only two or three
determinations, enough to check out the error, are necessary for the constant
factors, altitude, slope, exposure, and surface. An account of the exact
procedure employed in class study at Lincoln and Minnehaha will best
serve to illustrate the use of this method. The series of stations chosen at
Lincoln were primarily within a single formation, for the purpose of
determining the physical factor variation in different areas. One series was
located in the prairie-grass formation (Koelera-Andropogon-psilium), and
consisted of the following stations: (1) low prairie, (2) crest of ridge I, (3)
northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of ridge II,
(6) bare crest of ridge II, (7) thicket ravine. The other series was established
in the bur-oak-hickory forest (Quercus-Hicoria-hylium) at the following
Fig. 29. Series of stations: I, at Minnehaha; II, at Lincoln in the prairie formation.
132. Choice of stations. This method is based upon simultaneous
readings by means of simple instruments in a series of habitats, or of
stations in a single habitat. Such readings are necessary for the variable
atmospheric factors, humidity, light, temperature, and wind. Frequent
readings suffice for water-content and precipitation, while only two or three
determinations, enough to check out the error, are necessary for the constant
factors, altitude, slope, exposure, and surface. An account of the exact
procedure employed in class study at Lincoln and Minnehaha will best
serve to illustrate the use of this method. The series of stations chosen at
Lincoln were primarily within a single formation, for the purpose of
determining the physical factor variation in different areas. One series was
located in the prairie-grass formation (Koelera-Andropogon-psilium), and
consisted of the following stations: (1) low prairie, (2) crest of ridge I, (3)
northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of ridge II,
(6) bare crest of ridge II, (7) thicket ravine. The other series was established
in the bur-oak-hickory forest (Quercus-Hicoria-hylium) at the following
Page 127
stations: (1) thicket, (2) woodland, (3) knoll in forest, (4) depression in
forest, (5) level forest floor, (6) nettle thicket, (7) brook bank. At
Minnehaha the series was primarily one of different formations: (1) the pine
formation (Pinus-xerohylium), (2) the gravel slide formation
(Pseudocymopterus-Mentzelia-chalicium), (3) east slope of spruce forest
(Picea-Pseudotsuga-hylium), (4) ridge in the spruce forest, (5) north slope
of spruce forest, (6) brook bank in forest, (7) the thicket formation
(Quercus-Cercocarpus-lochmodium), (8) the aspen formation (Populus-
hylium). When permanent or temporary quadrats are established, they are
ordinarily used as regular stations, since this enables one to refer the
physical factor readings to a few definite individual plants, as well as to the
entire formation. The transects in figure 29 illustrate two of the above series
of stations.
133. Time of readings. The frequency of simple readings and the times
at which they are made must be regulated largely by opportunity and
convenience. In addition to making readings once or twice a week
throughout the season, the series should be read at least once every day for
a representative week or two. It is also very desirable to have a series for
each hour of a typical day, or of two days, one of which is clear, the other
cloudy. When a single daily reading is made, it should be taken at or as near
meridian as possible. The usual series is the one obtained by simultaneous
observations at the same level in different stations. An important series is
also secured by simultaneous readings at the various levels of the same
station, though it is not necessary to take this series frequently.
forest, (5) level forest floor, (6) nettle thicket, (7) brook bank. At
Minnehaha the series was primarily one of different formations: (1) the pine
formation (Pinus-xerohylium), (2) the gravel slide formation
(Pseudocymopterus-Mentzelia-chalicium), (3) east slope of spruce forest
(Picea-Pseudotsuga-hylium), (4) ridge in the spruce forest, (5) north slope
of spruce forest, (6) brook bank in forest, (7) the thicket formation
(Quercus-Cercocarpus-lochmodium), (8) the aspen formation (Populus-
hylium). When permanent or temporary quadrats are established, they are
ordinarily used as regular stations, since this enables one to refer the
physical factor readings to a few definite individual plants, as well as to the
entire formation. The transects in figure 29 illustrate two of the above series
of stations.
133. Time of readings. The frequency of simple readings and the times
at which they are made must be regulated largely by opportunity and
convenience. In addition to making readings once or twice a week
throughout the season, the series should be read at least once every day for
a representative week or two. It is also very desirable to have a series for
each hour of a typical day, or of two days, one of which is clear, the other
cloudy. When a single daily reading is made, it should be taken at or as near
meridian as possible. The usual series is the one obtained by simultaneous
observations at the same level in different stations. An important series is
also secured by simultaneous readings at the various levels of the same
station, though it is not necessary to take this series frequently.
Page 128
Fig. 30. A denuded station in the aspen formation.
134. Details of the method. After the stations have been selected by a
careful preliminary survey of the habitat or series of habitats, their location
is indicated by a small flag bearing a number, in case there is no danger of
these being disturbed. Otherwise, less conspicuous stakes are used. The
ordinary practice is to visit each station of the series, and to take readings of
water-content, altitude, slope, and exposure. On the first trip these are all
made by the instructor, but after a short time the determination of each
factor may be assigned in rotation to each of the students. After these
constant factors have been read and recorded, one student is equipped with
photometer, thermometer, and psychrometer, and, if desirable, anemometer,
and left at the first station. At each succeeding station the same plan is
followed, so that at the end of the series the constant factors have all been
read, and there is an observer at each station prepared to make readings of
the variable ones. The task of acquainting the students with the operation of
photometer, psychrometer, etc., can best be done in class or at a previous
134. Details of the method. After the stations have been selected by a
careful preliminary survey of the habitat or series of habitats, their location
is indicated by a small flag bearing a number, in case there is no danger of
these being disturbed. Otherwise, less conspicuous stakes are used. The
ordinary practice is to visit each station of the series, and to take readings of
water-content, altitude, slope, and exposure. On the first trip these are all
made by the instructor, but after a short time the determination of each
factor may be assigned in rotation to each of the students. After these
constant factors have been read and recorded, one student is equipped with
photometer, thermometer, and psychrometer, and, if desirable, anemometer,
and left at the first station. At each succeeding station the same plan is
followed, so that at the end of the series the constant factors have all been
read, and there is an observer at each station prepared to make readings of
the variable ones. The task of acquainting the students with the operation of
photometer, psychrometer, etc., can best be done in class or at a previous
Page 129
field period, as it is evident that they must be familiar with the instruments
before they can use them accurately in the field series. The details of
operation have already been given and need not be repeated here. The task
of obtaining readings at the same moment may be met by supplying each
observer with a watch, which runs exactly with all the others, or by making
observations upon signal. The second means has been found most
successful in practice, since the signal fixes the attention at the exact
moment. The best plan is for the instructor to occupy a commanding
position somewhere near the middle of the series, and to give the signals by
shout or whistle at the proper interval. Considerable care and experience are
necessary to do the last satisfactorily. Sufficient time must be given for the
operation of the instrument and the making of the record. In addition, a
period must be permitted to elapse which is long enough for every
instrument to reach the proper reading. For example, in a series which
contains a gravel slide and a forest, the thermometer which has just been
used for an air reading will require four or five times as long an interval to
respond to the temperature of the gravel as to that of the cool forest floor. In
such series, the instructor should regularly take his place in the station
where the response is slowest or greatest. He must record the exact time of
each signal, and note any general changes of sky or wind that produce
temporary fluctuations at the time of reading. When the readings extend
over a whole day, the usual plan is to begin at the last station and take a
second series of water-content samples, noting the exact time in order that
the rate of water loss may be determined. A check series of physiographic
factors may be made at this time also, or this may be left for future visits.
While it is unnecessary to take soil samples oftener than once a day, it is
important to make at least one series at each visit. Sometimes it becomes
desirable to know the rate of water loss in different stations during the day,
and in this event, samples are taken at one or two hour intervals for the
entire day.
In making simultaneous readings at the different levels of one station, the
observers are grouped in one spot in such a way that they do not interfere
with the correct reading of each instrument. Readings of this sort are most
valuable in the case of temperature, which shows greater differences at the
various levels. Important differences of humidity and wind also are readily
obtained, and, in layered formations, marked variations in the amount of
light. In the open, the ordinary levels for temperature are 6 feet, 3 feet,
before they can use them accurately in the field series. The details of
operation have already been given and need not be repeated here. The task
of obtaining readings at the same moment may be met by supplying each
observer with a watch, which runs exactly with all the others, or by making
observations upon signal. The second means has been found most
successful in practice, since the signal fixes the attention at the exact
moment. The best plan is for the instructor to occupy a commanding
position somewhere near the middle of the series, and to give the signals by
shout or whistle at the proper interval. Considerable care and experience are
necessary to do the last satisfactorily. Sufficient time must be given for the
operation of the instrument and the making of the record. In addition, a
period must be permitted to elapse which is long enough for every
instrument to reach the proper reading. For example, in a series which
contains a gravel slide and a forest, the thermometer which has just been
used for an air reading will require four or five times as long an interval to
respond to the temperature of the gravel as to that of the cool forest floor. In
such series, the instructor should regularly take his place in the station
where the response is slowest or greatest. He must record the exact time of
each signal, and note any general changes of sky or wind that produce
temporary fluctuations at the time of reading. When the readings extend
over a whole day, the usual plan is to begin at the last station and take a
second series of water-content samples, noting the exact time in order that
the rate of water loss may be determined. A check series of physiographic
factors may be made at this time also, or this may be left for future visits.
While it is unnecessary to take soil samples oftener than once a day, it is
important to make at least one series at each visit. Sometimes it becomes
desirable to know the rate of water loss in different stations during the day,
and in this event, samples are taken at one or two hour intervals for the
entire day.
In making simultaneous readings at the different levels of one station, the
observers are grouped in one spot in such a way that they do not interfere
with the correct reading of each instrument. Readings of this sort are most
valuable in the case of temperature, which shows greater differences at the
various levels. Important differences of humidity and wind also are readily
obtained, and, in layered formations, marked variations in the amount of
light. In the open, the ordinary levels for temperature are 6 feet, 3 feet,
Page 130
surface, 5, 10, and 15 inches in the ground, and for wind and humidity, 6
feet, 3 feet, and surface. In forests the same levels are used for comparison
with formations in the open, but a more desirable series for light especially
is secured by making readings at the height of, or better, just below the
various layers. Series of this sort are likewise made on signal. The best time
of day is that of a period in which the middle station is read near meridian,
since the variation due to time is sufficiently small to permit fairly accurate
comparisons between the readings for the different stations.
135. Records. The form used for recording the observations made by
means of simple instruments is shown below. It is hardly necessary to state
that it may be readily modified to suit the needs of different investigators.
Ordinarily, each sheet is used for the records of one habitat or series alone,
but for convenience sake, the records of two different series are here
combined. The figures given are taken from records for the prairie and
forest formations at Lincoln.
Koelera-Andropogon-psilium
April 25, 1901. Clear. South wind.
feet, 3 feet, and surface. In forests the same levels are used for comparison
with formations in the open, but a more desirable series for light especially
is secured by making readings at the height of, or better, just below the
various layers. Series of this sort are likewise made on signal. The best time
of day is that of a period in which the middle station is read near meridian,
since the variation due to time is sufficiently small to permit fairly accurate
comparisons between the readings for the different stations.
135. Records. The form used for recording the observations made by
means of simple instruments is shown below. It is hardly necessary to state
that it may be readily modified to suit the needs of different investigators.
Ordinarily, each sheet is used for the records of one habitat or series alone,
but for convenience sake, the records of two different series are here
combined. The figures given are taken from records for the prairie and
forest formations at Lincoln.
Koelera-Andropogon-psilium
April 25, 1901. Clear. South wind.
Page 131
TEMPERATURE LIGHT HUMIDITY WATER-CONTENT WIND
TIME 3:20 3:24 3:30 3:35 3:40 3:45 3:55 % 2:40 3:05
P.M.
STATION 1½ m. Surf. 5 10 15 1½ m. Surf. 1½ m. Surf. 5 10 15 1½ m. Surf.
1. 27.8 29.6 17.8 15.8 10.9 57 59 17.7 14.6 17.4 740 280
2. 26.5 31.3 18.3 16.6 12.8 59 59 17.9 12.2 14.3 1100 510
3. 26.9 28.5 18.2 14.2 13.5 58 59 16.5 16.9 19 980 520
4. 26.2 30 16.8 13.2 11.6 63 66 24.4 21.3 24.8 920 460
5. 28 32.4 18.6 14.6 14.2 59 60 10.7 17 17.2 1080 490
6. 28 40.8 23.8 16 15 51 51 5 8.3 10.3 1010 410
7. 26.4 30.8 16 13 11 68 70 27 24.3 21.4 680 52
Quercus-Hicoria-hylium
April 20, 1901. Clear. Southeast wind.
TIME 3:20 3:24 3:30 3:35 3:40 3:45 3:55 % 2:40 3:05
P.M.
STATION 1½ m. Surf. 5 10 15 1½ m. Surf. 1½ m. Surf. 5 10 15 1½ m. Surf.
1. 27.8 29.6 17.8 15.8 10.9 57 59 17.7 14.6 17.4 740 280
2. 26.5 31.3 18.3 16.6 12.8 59 59 17.9 12.2 14.3 1100 510
3. 26.9 28.5 18.2 14.2 13.5 58 59 16.5 16.9 19 980 520
4. 26.2 30 16.8 13.2 11.6 63 66 24.4 21.3 24.8 920 460
5. 28 32.4 18.6 14.6 14.2 59 60 10.7 17 17.2 1080 490
6. 28 40.8 23.8 16 15 51 51 5 8.3 10.3 1010 410
7. 26.4 30.8 16 13 11 68 70 27 24.3 21.4 680 52
Quercus-Hicoria-hylium
April 20, 1901. Clear. Southeast wind.
Page 132
TEMPERATURE LIGHT HUMIDITY WATER-CONTENT WIND
TIME 10:40 10:46 10:50 10:55 11:00 12:00 12:05 11:10 11:20 % 11:30 11:45
A.M.
STATION 1½ m. Surf. 5 10 15 1½ m. Surf. 1½ m. Surf. 5 10 15 1½ m. Surf.
1. 16 25 9.6 8.4 7.8 .08 .06 73 81 24.2 19.2 19.5 298 0
2. 16.2 30.5 8.5 8.4 7.8 .11 .09 73 86 22 22.5 19.4 375 2
3. 16.2 17.8 7.6 7.8 8 .08 .06 73 95 22.1 20.4 21.6 640 6
4. 15.6 26.2 10.6 8.4 8.2 .06 .03 81 95 25.4 23.1 22.4 275 12
5. 17.6 25.4 7.6 7.4 7.2 .03 .02 90 95 27.2 19.8 18.8 178 2
6. 16.2 20.2 8.4 7 6.2 .02 .01 82 90 27.6 20.8 18.8 115 4
7. 15.8 17.2 6.4 6.4 6.1 .05 .04 82 90 23.8 19 19.3 60 0
TIME 10:40 10:46 10:50 10:55 11:00 12:00 12:05 11:10 11:20 % 11:30 11:45
A.M.
STATION 1½ m. Surf. 5 10 15 1½ m. Surf. 1½ m. Surf. 5 10 15 1½ m. Surf.
1. 16 25 9.6 8.4 7.8 .08 .06 73 81 24.2 19.2 19.5 298 0
2. 16.2 30.5 8.5 8.4 7.8 .11 .09 73 86 22 22.5 19.4 375 2
3. 16.2 17.8 7.6 7.8 8 .08 .06 73 95 22.1 20.4 21.6 640 6
4. 15.6 26.2 10.6 8.4 8.2 .06 .03 81 95 25.4 23.1 22.4 275 12
5. 17.6 25.4 7.6 7.4 7.2 .03 .02 90 95 27.2 19.8 18.8 178 2
6. 16.2 20.2 8.4 7 6.2 .02 .01 82 90 27.6 20.8 18.8 115 4
7. 15.8 17.2 6.4 6.4 6.1 .05 .04 82 90 23.8 19 19.3 60 0
Page 133
METHOD OF ECOGRAPH BATTERIES
136. A battery of recording instruments consists of a selagraph, a
psychrograph, and a thermograph, to which an anemograph is added when
possible. As stated before, the determination of water-content by the
geotome method is more satisfactory than by any automatic instrument yet
devised. When the base station is located where the sunlight is
unobstructed, which should be the case whenever possible, it is unnecessary
to include a selagraph in those batteries placed in similarly exposed
stations, since the light values will be the same. As a rule, batteries are
established within different zones or different habitats, except where a
highly diversified habitat is made the subject of special inquiry. Such a
restriction arises from the fact that expense, care of operation, etc., place a
limit upon the number of batteries, and, in such case, the task of primary
importance is to establish the physical character of representative habitats.
For these reasons, the first series of thermographs established in 1903 was
located with respect to altitude, the instruments being placed at Manitou
2,000 m., Minnehaha, 2,600 m., and Mount Garfield 3,800 m. In 1904, the
stations established for the record of temperature and humidity were
situated with respect to habitats representing the four formations: gravel
slide, half gravel slide, spruce forest, and brook bank.
The batteries are located and set up according to the directions already
given. A 2–meter quadrat with the battery as the middle is staked and
mapped. Within this, all readings of water-content, soil temperature, and
physiographic factors are made. Altitude, slope, exposure, and cover are
recorded when each battery is located, and a soil sample is taken for
mechanical analysis. When the position of the batteries permits it, water-
content readings should be made frequently, once or twice a week at least.
In addition, a complete series of samples should be taken daily for a period
sufficient to indicate the ordinary extremes of water-content.
The ecograph battery of each habitat constitutes a standard to which the
results obtained by simple instruments may be referred with accuracy. It not
only does this, but it also serves as a basis for interpreting the readings of
simple instruments in distant habitats of the same character. In this way a
few batteries judiciously placed make possible the exact physical
136. A battery of recording instruments consists of a selagraph, a
psychrograph, and a thermograph, to which an anemograph is added when
possible. As stated before, the determination of water-content by the
geotome method is more satisfactory than by any automatic instrument yet
devised. When the base station is located where the sunlight is
unobstructed, which should be the case whenever possible, it is unnecessary
to include a selagraph in those batteries placed in similarly exposed
stations, since the light values will be the same. As a rule, batteries are
established within different zones or different habitats, except where a
highly diversified habitat is made the subject of special inquiry. Such a
restriction arises from the fact that expense, care of operation, etc., place a
limit upon the number of batteries, and, in such case, the task of primary
importance is to establish the physical character of representative habitats.
For these reasons, the first series of thermographs established in 1903 was
located with respect to altitude, the instruments being placed at Manitou
2,000 m., Minnehaha, 2,600 m., and Mount Garfield 3,800 m. In 1904, the
stations established for the record of temperature and humidity were
situated with respect to habitats representing the four formations: gravel
slide, half gravel slide, spruce forest, and brook bank.
The batteries are located and set up according to the directions already
given. A 2–meter quadrat with the battery as the middle is staked and
mapped. Within this, all readings of water-content, soil temperature, and
physiographic factors are made. Altitude, slope, exposure, and cover are
recorded when each battery is located, and a soil sample is taken for
mechanical analysis. When the position of the batteries permits it, water-
content readings should be made frequently, once or twice a week at least.
In addition, a complete series of samples should be taken daily for a period
sufficient to indicate the ordinary extremes of water-content.
The ecograph battery of each habitat constitutes a standard to which the
results obtained by simple instruments may be referred with accuracy. It not
only does this, but it also serves as a basis for interpreting the readings of
simple instruments in distant habitats of the same character. In this way a
few batteries judiciously placed make possible the exact physical
Page 134
investigation of a large number of habitats, covering a considerable area.
The only limit, indeed, upon this method is that placed by time. The
proportionate use of batteries and of simple instruments must be largely
determined by the conditions which confront the investigator. It is obvious
that, where expense is not a decisive factor, the gain in time and in
completeness of results is enormously in favor of the battery. There is an
additional value in the automatic and continuous record which can not be
overlooked. When the use of instruments in the study of habitat and
formation becomes universal, the importance of the ecograph will be
immeasurably enhanced. It will be possible to secure duplicate records of
batteries located in the most remote and diverse regions, from the equator to
the poles, and comparative phytogeography upon a scientific basis will for
the first time be possible. This opens an alluring vista of the future when
ecologists the world over will cooperate in such a way that the results
obtained by ecograph batteries anywhere on the globe will permit of exact
comparison.
The only limit, indeed, upon this method is that placed by time. The
proportionate use of batteries and of simple instruments must be largely
determined by the conditions which confront the investigator. It is obvious
that, where expense is not a decisive factor, the gain in time and in
completeness of results is enormously in favor of the battery. There is an
additional value in the automatic and continuous record which can not be
overlooked. When the use of instruments in the study of habitat and
formation becomes universal, the importance of the ecograph will be
immeasurably enhanced. It will be possible to secure duplicate records of
batteries located in the most remote and diverse regions, from the equator to
the poles, and comparative phytogeography upon a scientific basis will for
the first time be possible. This opens an alluring vista of the future when
ecologists the world over will cooperate in such a way that the results
obtained by ecograph batteries anywhere on the globe will permit of exact
comparison.
Page 135
THE EXPRESSION OF PHYSICAL FACTOR RESULTS
137. The form of results. It is almost inevitable that the general adoption
of precise methods of measuring the habitat will result in a common form
for expressing the physical character of the latter. An actual diagnosis of
each habitat is not a difficult matter, after the factors are carefully
measured, and will unquestionably lead to very desirable definiteness and
precision. The accurate investigation of the physical factors of a number of
habitats for one growing season furnishes the necessary material for a
diagnosis based upon the mean for the growing season. Similar results for
two or three seasons will yield a diagnosis as accurate and as final as that of
a formation, or, indeed, as that of many species. The author’s investigations
have not yet gone far enough to warrant proposing a final form for this, but
the following diagnosis is offered as a suggestion:
Elymus-Muhlenbergia-chalicium. Habitat: holard 9 per cent, chresard 8
per cent, relative humidity 40 per cent, light 0.6, soil colluvial gravel
(gravel 70 per cent, sand 27 per cent, silt 3 per cent), air temperature 65°,
surface 82°, soil 59°, wind 10 miles, rainfall 8 inches, altitude 2,800 m.,
slope 23°, exposure south, surface even, cover open, no active biotic
agencies.
The detailed comparison of habitats is made most readily by the graphic
method of curves, which constitute the most desirable form of expression in
connection with the original record upon which they are based. Factor
means are particularly desirable for diagnostic purposes, and they furnish
valuable curves also. Factor sums are impracticable at present, and it seems
doubtful that they will ever be of much value. It is by no means impossible,
however, that a more detailed and exact knowledge of the physiology of
adaptation, coupled with methods of precision in the habitat, will render
them necessary.
Factor Records
138. Experience has shown that the practice of making hasty and often
formless records in the field is unwise and is apt to be inaccurate as well.
137. The form of results. It is almost inevitable that the general adoption
of precise methods of measuring the habitat will result in a common form
for expressing the physical character of the latter. An actual diagnosis of
each habitat is not a difficult matter, after the factors are carefully
measured, and will unquestionably lead to very desirable definiteness and
precision. The accurate investigation of the physical factors of a number of
habitats for one growing season furnishes the necessary material for a
diagnosis based upon the mean for the growing season. Similar results for
two or three seasons will yield a diagnosis as accurate and as final as that of
a formation, or, indeed, as that of many species. The author’s investigations
have not yet gone far enough to warrant proposing a final form for this, but
the following diagnosis is offered as a suggestion:
Elymus-Muhlenbergia-chalicium. Habitat: holard 9 per cent, chresard 8
per cent, relative humidity 40 per cent, light 0.6, soil colluvial gravel
(gravel 70 per cent, sand 27 per cent, silt 3 per cent), air temperature 65°,
surface 82°, soil 59°, wind 10 miles, rainfall 8 inches, altitude 2,800 m.,
slope 23°, exposure south, surface even, cover open, no active biotic
agencies.
The detailed comparison of habitats is made most readily by the graphic
method of curves, which constitute the most desirable form of expression in
connection with the original record upon which they are based. Factor
means are particularly desirable for diagnostic purposes, and they furnish
valuable curves also. Factor sums are impracticable at present, and it seems
doubtful that they will ever be of much value. It is by no means impossible,
however, that a more detailed and exact knowledge of the physiology of
adaptation, coupled with methods of precision in the habitat, will render
them necessary.
Factor Records
138. Experience has shown that the practice of making hasty and often
formless records in the field is unwise and is apt to be inaccurate as well.
Page 136
The time saved in the field is more than counterbalanced by that consumed
in copying the results into the permanent form. The danger of error in field
notes rapidly taken is very grave, and the chance of confusion and the waste
of time in deciphering them are great. Moreover, the task of checking a
copy with the original, which is absolutely necessary for accuracy, involves
a further expenditure of time and energy. For these reasons the field record
should be made in permanent form. Definite record sheets are used, and the
invariable rule is made that all readings are to be noted in ink at the time
and spot where they are taken. On a long journey, or in the face of many
observations, the tendency to take notes or to record observations rapidly is
very great, but this will correct itself after a few attempts to use such notes.
The record forms for various factors have been indicated in the proper
place, as well as the one for simultaneous readings. Ecograph sheets are
carefully filed, and constitute permanent records. With a little practice they
may be read almost as easily as tables, and any attempt to put them into
tabular form is a mere waste of time. For purposes of study and of
publication, it often becomes necessary to bring together all the results
obtained for a particular habitat, both by simple instruments and by
ecographs. The form of record used for this is essentially that already
indicated for simultaneous readings on page 92, since general features and
constant factors can not well be included in the table. Record sheets of this
type have been printed at a cost of $5 per thousand, and the various factor
records can be obtained at about the same rate. The size of sheet used is 9½
× 7¾ inches. The record book is the usual notebook cover, which has been
found neither too large nor too small. It is protected from dirt and rain by a
covering of oilcloth which overlaps the edges. Record books should be
carefully labeled, and each one should contain a single year’s records.
Factor Curves
139. Plotting. The paper employed is divided into centimeter squares
which are subdivided into 2–millimeter units. For ordinary curves the size
of sheet is 9½ × 7¾ inches, which makes it possible for curve sheets to be
filed in the record book. Tablets containing 60 of these sheets can be
obtained for 20 cents each from the Central School Supply House, Chicago.
For curves longer than 9 inches special sizes of sheets must be used. On
in copying the results into the permanent form. The danger of error in field
notes rapidly taken is very grave, and the chance of confusion and the waste
of time in deciphering them are great. Moreover, the task of checking a
copy with the original, which is absolutely necessary for accuracy, involves
a further expenditure of time and energy. For these reasons the field record
should be made in permanent form. Definite record sheets are used, and the
invariable rule is made that all readings are to be noted in ink at the time
and spot where they are taken. On a long journey, or in the face of many
observations, the tendency to take notes or to record observations rapidly is
very great, but this will correct itself after a few attempts to use such notes.
The record forms for various factors have been indicated in the proper
place, as well as the one for simultaneous readings. Ecograph sheets are
carefully filed, and constitute permanent records. With a little practice they
may be read almost as easily as tables, and any attempt to put them into
tabular form is a mere waste of time. For purposes of study and of
publication, it often becomes necessary to bring together all the results
obtained for a particular habitat, both by simple instruments and by
ecographs. The form of record used for this is essentially that already
indicated for simultaneous readings on page 92, since general features and
constant factors can not well be included in the table. Record sheets of this
type have been printed at a cost of $5 per thousand, and the various factor
records can be obtained at about the same rate. The size of sheet used is 9½
× 7¾ inches. The record book is the usual notebook cover, which has been
found neither too large nor too small. It is protected from dirt and rain by a
covering of oilcloth which overlaps the edges. Record books should be
carefully labeled, and each one should contain a single year’s records.
Factor Curves
139. Plotting. The paper employed is divided into centimeter squares
which are subdivided into 2–millimeter units. For ordinary curves the size
of sheet is 9½ × 7¾ inches, which makes it possible for curve sheets to be
filed in the record book. Tablets containing 60 of these sheets can be
obtained for 20 cents each from the Central School Supply House, Chicago.
For curves longer than 9 inches special sizes of sheets must be used. On
Page 137
account of their inconvenience large sheets are avoided whenever possible.
This can usually be accomplished by increasing the numerical value of the
intervals. The inks employed in plotting are the waterproof inks of Chas.
Higgins & Co., Brooklyn, New York. These are made in ten or more colors,
black, violet, indigo, blue, green, yellow, orange, brown, brick red, carmine,
and scarlet, and cost 25 cents per bottle. In addition to being waterproof,
they make it possible to combine curves in all conceivable ways without
destroying their identity. Furthermore, it is a great advantage to use the
same color invariably for the same kind of curve: thus, it has been the
practice to indicate the 3–foot, surface, 5, 10, and 15–inch temperature
curves by violet, green, yellow, blue, and carmine respectively. A fine-
pointed pen, such as the Spencerian No. 1, is most satisfactory for inking;
drawing pens, such as Gillott’s Crowquill, are too finely pointed for
ordinary use.
In plotting a curve, it is first necessary to determine the value of the
interval, and the extreme range of the curve or combination. For example,
in the case of temperature, it is most convenient to assign a value of 1°
Centigrade to each centimeter, since the thermometers used read to one-fifth
of a degree, which corresponds exactly to the 2–millimeter units of each
square. The length of the sheet permits a range of 22 degrees Centigrade,
and the actual limits must be determined for the particular results to be
employed. For the same region, it is very desirable that the unit interval and
the range be the same, in order that all curve sheets may admit of direct
comparison. Indeed, it is greatly to be hoped that in the future ecologists
will agree to a uniform system of curve-plotting, cartography, etc., as the
geographers are beginning to do in the construction of maps. The major
intervals are written, or, better, typewritten, at both sides of the sheet, and
the time or space intervals are indicated at the top. Each curve sheet is
properly labeled, and essential data indicated. The readings are taken from
the field record, and their proper positions indicated by a dot. These are
connected first by a pencil line, the curves being made abrupt rather than
flowing; and the line, after having been carefully checked, is traced in ink.
140. Kinds of curves. Curves are named both with reference to the factor
concerned and the position or sequence of the readings. The factors which
lend themselves most readily to this method of representation are the
variable ones, water-content, humidity, light, temperature, and wind, and
corresponding curves are distinguished. Altitude and slope may likewise be
This can usually be accomplished by increasing the numerical value of the
intervals. The inks employed in plotting are the waterproof inks of Chas.
Higgins & Co., Brooklyn, New York. These are made in ten or more colors,
black, violet, indigo, blue, green, yellow, orange, brown, brick red, carmine,
and scarlet, and cost 25 cents per bottle. In addition to being waterproof,
they make it possible to combine curves in all conceivable ways without
destroying their identity. Furthermore, it is a great advantage to use the
same color invariably for the same kind of curve: thus, it has been the
practice to indicate the 3–foot, surface, 5, 10, and 15–inch temperature
curves by violet, green, yellow, blue, and carmine respectively. A fine-
pointed pen, such as the Spencerian No. 1, is most satisfactory for inking;
drawing pens, such as Gillott’s Crowquill, are too finely pointed for
ordinary use.
In plotting a curve, it is first necessary to determine the value of the
interval, and the extreme range of the curve or combination. For example,
in the case of temperature, it is most convenient to assign a value of 1°
Centigrade to each centimeter, since the thermometers used read to one-fifth
of a degree, which corresponds exactly to the 2–millimeter units of each
square. The length of the sheet permits a range of 22 degrees Centigrade,
and the actual limits must be determined for the particular results to be
employed. For the same region, it is very desirable that the unit interval and
the range be the same, in order that all curve sheets may admit of direct
comparison. Indeed, it is greatly to be hoped that in the future ecologists
will agree to a uniform system of curve-plotting, cartography, etc., as the
geographers are beginning to do in the construction of maps. The major
intervals are written, or, better, typewritten, at both sides of the sheet, and
the time or space intervals are indicated at the top. Each curve sheet is
properly labeled, and essential data indicated. The readings are taken from
the field record, and their proper positions indicated by a dot. These are
connected first by a pencil line, the curves being made abrupt rather than
flowing; and the line, after having been carefully checked, is traced in ink.
140. Kinds of curves. Curves are named both with reference to the factor
concerned and the position or sequence of the readings. The factors which
lend themselves most readily to this method of representation are the
variable ones, water-content, humidity, light, temperature, and wind, and
corresponding curves are distinguished. Altitude and slope may likewise be
Page 138
shown by means of curves, but the use of cross section or contour lines
serves the same purpose and is more natural. With regard to time and
position, curves are distinguished as level, station, and point curves. A level
curve is one based upon readings made at the same level through a series of
stations or of habitats, e. g., the level curve of surface temperature. The
station curve represents the various levels or points at which readings are
made in a single station. The point curve has for a basis the hourly or daily
variation of a factor at a particular point or level in a station. All of these
may be simple curves, when established upon a single reading for a series,
or mean curves when they are based upon the mean of a number of
readings. Curves which show the extremes of a factor, i. e., the maximum
and minimum, are also extremely valuable, though a combination of the
two for comparison is preferable.
141. Combinations of curves are invaluable for bringing similar curves
together, and permitting ready comparison of them. For this, and also
because they save space, they are regularly employed to the almost
complete exclusion of single curves. Combinations are made simply by
tracing the curves to be compared upon the same sheet, it being understood
that dissimilar curves, e. g., level and station, can not be combined. Colored
inks are an absolute necessity in combining; the primary principle
underlying their use is that curves that approach closely or cross each other
must be traced in inks that contrast sharply. As elsewhere stated, it has been
made the invariable rule to use the same color for the same level or point.
This applies especially to temperature, but holds also for humidity, light,
wind, and water-content, so that the color always indicates the level. For the
same reason, it is applied to a combination of point curves for one station,
though it is inapplicable to a series of point curves when these lie in the
same level. Light readings above 6 feet and water-content readings below
15 inches necessitate the use of additional colors.
Combinations may be made of the curves of a single factor for purposes
of comparison, or they may consist of curves of different factors in order to
aid in interpreting or indicating their relation to each other. Curves of the
same factor may be combined to form various series. The level series
consists of all the level curves for the stations under observation, e. g., the
six levels for temperature, three levels for wind, etc. Similarly, the station
series is a combination of all the station curves, and a corresponding
arrangement may be made for point curves with reference either to station
serves the same purpose and is more natural. With regard to time and
position, curves are distinguished as level, station, and point curves. A level
curve is one based upon readings made at the same level through a series of
stations or of habitats, e. g., the level curve of surface temperature. The
station curve represents the various levels or points at which readings are
made in a single station. The point curve has for a basis the hourly or daily
variation of a factor at a particular point or level in a station. All of these
may be simple curves, when established upon a single reading for a series,
or mean curves when they are based upon the mean of a number of
readings. Curves which show the extremes of a factor, i. e., the maximum
and minimum, are also extremely valuable, though a combination of the
two for comparison is preferable.
141. Combinations of curves are invaluable for bringing similar curves
together, and permitting ready comparison of them. For this, and also
because they save space, they are regularly employed to the almost
complete exclusion of single curves. Combinations are made simply by
tracing the curves to be compared upon the same sheet, it being understood
that dissimilar curves, e. g., level and station, can not be combined. Colored
inks are an absolute necessity in combining; the primary principle
underlying their use is that curves that approach closely or cross each other
must be traced in inks that contrast sharply. As elsewhere stated, it has been
made the invariable rule to use the same color for the same level or point.
This applies especially to temperature, but holds also for humidity, light,
wind, and water-content, so that the color always indicates the level. For the
same reason, it is applied to a combination of point curves for one station,
though it is inapplicable to a series of point curves when these lie in the
same level. Light readings above 6 feet and water-content readings below
15 inches necessitate the use of additional colors.
Combinations may be made of the curves of a single factor for purposes
of comparison, or they may consist of curves of different factors in order to
aid in interpreting or indicating their relation to each other. Curves of the
same factor may be combined to form various series. The level series
consists of all the level curves for the stations under observation, e. g., the
six levels for temperature, three levels for wind, etc. Similarly, the station
series is a combination of all the station curves, and a corresponding
arrangement may be made for point curves with reference either to station
Page 139
or to level. An extremely valuable combination of curves is that of the
holard and chresard for a series of stations. The most important
combinations of the curves of different factors are naturally those based
upon factors intimately related to each other or to the plant. The grouping of
water-content and humidity curves is of great value, especially when the
transpiration curve is added. Light and temperature curves make an
interesting combination, while a humidity, temperature, and wind series is
of much aid in tracing the connection between these factors. Finally, it is
altogether feasible to arrange the curves of water-content, humidity, light,
temperature, and wind upon the same sheet in such fashion as to give a
graphic representation of the whole physical nature of a single habitat or a
series. In all combinations of curves representing different factors, it must
be borne in mind that the position of a curve does not represent a definite
value with reference to the others, since some are based upon per cents,
others upon degrees, etc. The comparison must be based upon the character
of the curves, but even then it is an important aid. An instructive grouping
has been employed where series of readings on the same day, or on two
successive days in forest and in prairie have yielded the usual level series of
curves. The series for the two habitats are arranged on the same page, one at
the right and the other at the left, and permit direct comparison of
corresponding level or factor curves, both with respect to position and
character.
142. The amplitude of all the curves described above is determined by
the unit values of the factors concerned, while the length is dependent upon
the number of stations, points, or times. The value assigned the latter upon
the plotting paper is purely arbitrary, but it is most convenient to fix this at
the centimeter square. The unit value for temperature is 1° Centigrade per
square, each subdivision of the latter representing 0.2, and the range being
22 degrees. For water-content curves, each square represents a value of 2
per cent, the smaller square being 0.4 per cent, and the range 2–48 per cent.
The unit value for humidity is taken as 5 per cent, making each small square
1 per cent, and giving room on the sheet for the entire range from 1–100 per
cent. Owing to the anemometer used, curves of wind velocity have been
based upon the number of feet per minute. One hundred feet is taken as the
unit value, and the range is from 0–2200 feet. The unit value for the curve
of light intensity is .005. Each small square is .001, which permits a range
from .001 to .01 on one sheet. Consequently, when it is desired to plot the
holard and chresard for a series of stations. The most important
combinations of the curves of different factors are naturally those based
upon factors intimately related to each other or to the plant. The grouping of
water-content and humidity curves is of great value, especially when the
transpiration curve is added. Light and temperature curves make an
interesting combination, while a humidity, temperature, and wind series is
of much aid in tracing the connection between these factors. Finally, it is
altogether feasible to arrange the curves of water-content, humidity, light,
temperature, and wind upon the same sheet in such fashion as to give a
graphic representation of the whole physical nature of a single habitat or a
series. In all combinations of curves representing different factors, it must
be borne in mind that the position of a curve does not represent a definite
value with reference to the others, since some are based upon per cents,
others upon degrees, etc. The comparison must be based upon the character
of the curves, but even then it is an important aid. An instructive grouping
has been employed where series of readings on the same day, or on two
successive days in forest and in prairie have yielded the usual level series of
curves. The series for the two habitats are arranged on the same page, one at
the right and the other at the left, and permit direct comparison of
corresponding level or factor curves, both with respect to position and
character.
142. The amplitude of all the curves described above is determined by
the unit values of the factors concerned, while the length is dependent upon
the number of stations, points, or times. The value assigned the latter upon
the plotting paper is purely arbitrary, but it is most convenient to fix this at
the centimeter square. The unit value for temperature is 1° Centigrade per
square, each subdivision of the latter representing 0.2, and the range being
22 degrees. For water-content curves, each square represents a value of 2
per cent, the smaller square being 0.4 per cent, and the range 2–48 per cent.
The unit value for humidity is taken as 5 per cent, making each small square
1 per cent, and giving room on the sheet for the entire range from 1–100 per
cent. Owing to the anemometer used, curves of wind velocity have been
based upon the number of feet per minute. One hundred feet is taken as the
unit value, and the range is from 0–2200 feet. The unit value for the curve
of light intensity is .005. Each small square is .001, which permits a range
from .001 to .01 on one sheet. Consequently, when it is desired to plot the
Page 140
curve of a series of habitats with a range in intensity greater than this it is
necessary to use a double sheet. This is the usual device when the range of
curves is too great, except where the excess is slight. In this case the curve
is left open at the top, and the value which the crest attains is indicated. All
curves in combination are labeled at the beginning or left to indicate the
level, station, or point, and at the end or right to show the time, or day, if
this is not the basis of the curve or series.
The discussion that precedes deals exclusively with curves representing
factors determined in the field. It applies with equal force to results
obtained by instruments in control houses. In these, however, all factors
except those directly experimented with, usually water-content and light,
are practically equalized, and the curves based upon them are used chiefly
to show how nearly equal they have become. The important curves are
those of the water-content series, both holard and chresard, and of the shade
tents. Where several houses are differentiated with respect to temperature or
humidity, curve series of both these factors are necessary.
Factor Means and Sums
143. It has been shown elsewhere that the daily mean of temperature can
be closely approximated from the maximum and minimum of both day and
night. Maximum-minimum instruments for the other factors are lacking,
however, and for light, humidity, and wind these values can only be
obtained from the ecograph which makes it possible to get the exact mean
from the sum of all the hour readings. When it comes to the seasonal mean,
the ecograph is even more necessary, exception being made for water-
content, in which case a number of readings on various days through the
season will suffice. The value of factor means for diagnosis and for curves
has already been sufficiently commented upon, and the feasibility of factor
sums already indicated.
necessary to use a double sheet. This is the usual device when the range of
curves is too great, except where the excess is slight. In this case the curve
is left open at the top, and the value which the crest attains is indicated. All
curves in combination are labeled at the beginning or left to indicate the
level, station, or point, and at the end or right to show the time, or day, if
this is not the basis of the curve or series.
The discussion that precedes deals exclusively with curves representing
factors determined in the field. It applies with equal force to results
obtained by instruments in control houses. In these, however, all factors
except those directly experimented with, usually water-content and light,
are practically equalized, and the curves based upon them are used chiefly
to show how nearly equal they have become. The important curves are
those of the water-content series, both holard and chresard, and of the shade
tents. Where several houses are differentiated with respect to temperature or
humidity, curve series of both these factors are necessary.
Factor Means and Sums
143. It has been shown elsewhere that the daily mean of temperature can
be closely approximated from the maximum and minimum of both day and
night. Maximum-minimum instruments for the other factors are lacking,
however, and for light, humidity, and wind these values can only be
obtained from the ecograph which makes it possible to get the exact mean
from the sum of all the hour readings. When it comes to the seasonal mean,
the ecograph is even more necessary, exception being made for water-
content, in which case a number of readings on various days through the
season will suffice. The value of factor means for diagnosis and for curves
has already been sufficiently commented upon, and the feasibility of factor
sums already indicated.
Page 141
CHAPTER III. THE PLANT
Stimulus and Response
GENERAL RELATIONS
144. The nature of stimuli. Whatever produces a change in the functions
of a plant is a stimulus. The latter may be a force or a material; it may be
imponderable or ponderable; effect, not character, determines a stimulus.
Consequently, reaction or response decides what constitutes a stimulus. The
presence of the latter can be recognized only through an appreciable or
visible response, since it is impossible to discriminate between an impact
which produces no reaction and one which produces a merely latent one.
From this it is evident that quantity is decisive in determining whether the
impact becomes a stimulus. Plants grow constantly under the influence of
many stimuli, all varying from time to time in amount. Small changes in
these are so frequent that, in many cases at least, the plant no longer
appreciably reacts to them. Such changes, though usually measurable, are
not stimuli. Furthermore, it must be clearly recognized that plants which are
in constant response to stimuli are stimulated anew by an efficient increase
or decrease in the amount of any one of these. As is well known, however,
such increase or decrease is a stimulus only within certain limits, and the
degree of change necessary to produce a response depends upon the amount
of the factor normally present. The entire absence of a force usually present,
moreover, often constitutes a stimulus, as is evident in the case of light. The
nature of the plant itself has a profound bearing upon the factors that act as
stimuli. Many species are extremely labile, and react strongly to relatively
slight stimuli; others are correspondingly stable, and respond only to stimuli
of much greater force. Some light is thrown upon the nature of this
difference by the behavior of ecads. A form which has grown under
Stimulus and Response
GENERAL RELATIONS
144. The nature of stimuli. Whatever produces a change in the functions
of a plant is a stimulus. The latter may be a force or a material; it may be
imponderable or ponderable; effect, not character, determines a stimulus.
Consequently, reaction or response decides what constitutes a stimulus. The
presence of the latter can be recognized only through an appreciable or
visible response, since it is impossible to discriminate between an impact
which produces no reaction and one which produces a merely latent one.
From this it is evident that quantity is decisive in determining whether the
impact becomes a stimulus. Plants grow constantly under the influence of
many stimuli, all varying from time to time in amount. Small changes in
these are so frequent that, in many cases at least, the plant no longer
appreciably reacts to them. Such changes, though usually measurable, are
not stimuli. Furthermore, it must be clearly recognized that plants which are
in constant response to stimuli are stimulated anew by an efficient increase
or decrease in the amount of any one of these. As is well known, however,
such increase or decrease is a stimulus only within certain limits, and the
degree of change necessary to produce a response depends upon the amount
of the factor normally present. The entire absence of a force usually present,
moreover, often constitutes a stimulus, as is evident in the case of light. The
nature of the plant itself has a profound bearing upon the factors that act as
stimuli. Many species are extremely labile, and react strongly to relatively
slight stimuli; others are correspondingly stable, and respond only to stimuli
of much greater force. Some light is thrown upon the nature of this
difference by the behavior of ecads. A form which has grown under
Page 142
comparatively uniform conditions for a long time seems to respond less
readily, and is therefore less labile than one which is subject to constant
fluctuation. In many cases this is not true, however, and the degree of
stability, i. e., of response, can only be connected in a general way with
taxonomic position.
145. The kinds of stimuli. The factors of a habitat are external to the
plant, and consequently are termed external stimuli. Properly speaking, all
stimuli are external, but since the response is often delayed or can not be
clearly traced, it may be permissible to speak of internal stimuli, i. e., those
which appear to originate within the plant. These, however, are extremely
obscure, and it is hardly possible to deal with them until much more is
known of the action of external stimuli. Of the latter, certain forces, gravity
and polarity, act in a way not at all understood, and as they are essentially
alike for all plants and all habitats, they can here be ignored. Stimuli are
imponderable when, like light and heat, they are measured with reference to
intensity, and ponderable, when, as in the case of water-content, humidity,
and salt-content, they can be expressed in mass or weight. It is undesirable
to insist upon this distinction, however, since the real character of a
stimulus is determined by its effect, and the latter is not necessarily
dependent upon whether the stimulus is one of force or one of material.
There is, however, a fundamental difference between factors with respect to
their relation to the plant. Direct factors alone are stimuli, since indirect
factors must always act through them. For example, the wind, its
mechanical influence excepted, can affect the plant only in so far as it is
converted into the stimulus of increased or decreased humidity.
Consequently, the normal stimuli of the plants of a formation are: (1) water-
content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. Soil,
pressure, physiography, and biotic factors influence plants only through
these, and are not stimuli, though exceptions must be made of biotic factors
in the case of sensitive, insectivorous, and gall-producing plants.
146. The nature of response. Since plants have no special organs for the
perception of stimuli, nor sensory tracts for their transmission, an external
stimulus acting upon a plant organ is ordinarily converted into a response at
once. The latter as a rule becomes evident immediately; in some cases it is
latent or imperceptible, or some time elapses before the chain of responses
finds visible expression. A marked decrease in humidity calls forth an
immediate increase of transpiration, but the ultimate response is seen in the
readily, and is therefore less labile than one which is subject to constant
fluctuation. In many cases this is not true, however, and the degree of
stability, i. e., of response, can only be connected in a general way with
taxonomic position.
145. The kinds of stimuli. The factors of a habitat are external to the
plant, and consequently are termed external stimuli. Properly speaking, all
stimuli are external, but since the response is often delayed or can not be
clearly traced, it may be permissible to speak of internal stimuli, i. e., those
which appear to originate within the plant. These, however, are extremely
obscure, and it is hardly possible to deal with them until much more is
known of the action of external stimuli. Of the latter, certain forces, gravity
and polarity, act in a way not at all understood, and as they are essentially
alike for all plants and all habitats, they can here be ignored. Stimuli are
imponderable when, like light and heat, they are measured with reference to
intensity, and ponderable, when, as in the case of water-content, humidity,
and salt-content, they can be expressed in mass or weight. It is undesirable
to insist upon this distinction, however, since the real character of a
stimulus is determined by its effect, and the latter is not necessarily
dependent upon whether the stimulus is one of force or one of material.
There is, however, a fundamental difference between factors with respect to
their relation to the plant. Direct factors alone are stimuli, since indirect
factors must always act through them. For example, the wind, its
mechanical influence excepted, can affect the plant only in so far as it is
converted into the stimulus of increased or decreased humidity.
Consequently, the normal stimuli of the plants of a formation are: (1) water-
content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. Soil,
pressure, physiography, and biotic factors influence plants only through
these, and are not stimuli, though exceptions must be made of biotic factors
in the case of sensitive, insectivorous, and gall-producing plants.
146. The nature of response. Since plants have no special organs for the
perception of stimuli, nor sensory tracts for their transmission, an external
stimulus acting upon a plant organ is ordinarily converted into a response at
once. The latter as a rule becomes evident immediately; in some cases it is
latent or imperceptible, or some time elapses before the chain of responses
finds visible expression. A marked decrease in humidity calls forth an
immediate increase of transpiration, but the ultimate response is seen in the
Page 143
closing of the stomata. A response to decreased light intensity, on the other
hand, is much less rapid and obvious. This difference is largely due to the
fact that the functional response is more marked, or at least more
perceptible in one case than in the other.
Response is the reaction of the plant to a stimulus; it begins with the
impact of an efficient factor, and ends only with the consequent final
readjustment. The immediate reaction is always functional. The nature and
intensity of the stimulus determine whether this functional response is
followed by a corresponding change in structure. The consideration of this
theme consequently gains in clearness if a functional and a structural
response be distinguished. The chief value of this distinction lies in the fact
that many reactions are functional alone; it serves also to emphasize the
absolute interdependence of structure and function, and the imperative need
of considering both in connection with the common stimulus. For these
reasons, the logical treatment is to connect each stimulus with its proper
functional change, and, through this, with the corresponding modification
of structure. For the sake of convenience, the term adjustment is used to
denote response in function, and adaptation, to indicate the response in
structure.
147. Adjustment and adaptation. The adjustment of a plant to the
stimuli of its habitat is a constant process. It is the daily task, seen in
nutrition and growth. So long as these take place under stimulation by
factors which fall within the normal variation of the habitat, the problems
belong to what has long been called physiology. When the stimuli become
unusual in degree or in kind, by a change of habitat or a modification in it,
adjustment is of much greater moment and is recorded in the plant’s
structure. These structural records are the foundation of proper ecological
study. Since they are the direct result of adjustment, however, this affords
further evidence that a division of the field into ecology and physiology is
illogical and superficial. Slight or periodical adjustment may concern
function alone; it may be expressed in the movement of parts or organs,
such as the closing of stomata or changes in the position of leaves, in
growth, or in modifications of structure. This expression is fundamentally
affected by the nature of the factor and is in direct relation to the intensity
of the latter. Adaptation comprises all structural changes resulting from
adjustment. It includes both growth and modification. The latter is merely
growth in response to unusual stimuli, but this fact is the real clue to all
hand, is much less rapid and obvious. This difference is largely due to the
fact that the functional response is more marked, or at least more
perceptible in one case than in the other.
Response is the reaction of the plant to a stimulus; it begins with the
impact of an efficient factor, and ends only with the consequent final
readjustment. The immediate reaction is always functional. The nature and
intensity of the stimulus determine whether this functional response is
followed by a corresponding change in structure. The consideration of this
theme consequently gains in clearness if a functional and a structural
response be distinguished. The chief value of this distinction lies in the fact
that many reactions are functional alone; it serves also to emphasize the
absolute interdependence of structure and function, and the imperative need
of considering both in connection with the common stimulus. For these
reasons, the logical treatment is to connect each stimulus with its proper
functional change, and, through this, with the corresponding modification
of structure. For the sake of convenience, the term adjustment is used to
denote response in function, and adaptation, to indicate the response in
structure.
147. Adjustment and adaptation. The adjustment of a plant to the
stimuli of its habitat is a constant process. It is the daily task, seen in
nutrition and growth. So long as these take place under stimulation by
factors which fall within the normal variation of the habitat, the problems
belong to what has long been called physiology. When the stimuli become
unusual in degree or in kind, by a change of habitat or a modification in it,
adjustment is of much greater moment and is recorded in the plant’s
structure. These structural records are the foundation of proper ecological
study. Since they are the direct result of adjustment, however, this affords
further evidence that a division of the field into ecology and physiology is
illogical and superficial. Slight or periodical adjustment may concern
function alone; it may be expressed in the movement of parts or organs,
such as the closing of stomata or changes in the position of leaves, in
growth, or in modifications of structure. This expression is fundamentally
affected by the nature of the factor and is in direct relation to the intensity
of the latter. Adaptation comprises all structural changes resulting from
adjustment. It includes both growth and modification. The latter is merely
growth in response to unusual stimuli, but this fact is the real clue to all
Page 144
evolution. Growth is periodic, and in a sense quantitative; it results from the
normal continuous adjustment of the plant to the stimuli of its proper
habitat. In contrast, modification is relatively permanent and qualitative; it
is the response to stimuli unusual in kind or intensity. A definite knowledge
of the processes of growth is indispensable to an understanding of
modification. In the fundamental task of connecting plant and habitat, it is
the modification of the plant, and not its growth, which records the
significant responses to stimuli. For this reason the discussion of adaptation
in the pages that follow is practically confined to modification of structure.
This is particularly desirable, since growth has long been the theme of
physiological study, while modification has too often been considered from
the structural standpoint alone. The comparatively few studies that have
taken function into account have been largely empirical; in them neither
stimulus nor adaptation has received anything approaching adequate
treatment.
148. The measurement of response. The amount of response to a
stimulus is proportional to the intensity of the factor concerned. This does
not mean that the same stimulus produces the same response in two distinct
species, or necessarily in two plants of one species. In these cases the rule
holds only when the plants or species are equally plastic. For each
individual, however, this quantitative correspondence of stimulus and
response is fundamental. It is uncertain whether an exact or constant ratio
can be established between factor and function; the answer to this must
await the general use of quantitative methods. There can be no doubt,
however, that within certain limits the adjustment is proportional to the
amount of stimulus, whereas reaction is well known to be abnormal or
inhibited beyond certain extremes. It is quite erroneous to think that
reaction is independent of quantity of stimulus, or to liken the stimulating
factor to “the smallest spark (which) by igniting a mass of powder, produces
an enormous mechanical effect.”[8] Such a statement is only apparently true
of the action of mechanical stimuli upon the few plants that may properly
be said to possess irritability, such as sensitive plants and certain
insectivorous ones. Of the normal relation of response to direct factors,
water, light, etc., it is entirely untrue. Axiomatically, there is ordinarily an
essential correspondence, also, between the amount of adjustment and of
adaptation. This correspondence is profoundly affected, however, by the
structural stability of the plant.
normal continuous adjustment of the plant to the stimuli of its proper
habitat. In contrast, modification is relatively permanent and qualitative; it
is the response to stimuli unusual in kind or intensity. A definite knowledge
of the processes of growth is indispensable to an understanding of
modification. In the fundamental task of connecting plant and habitat, it is
the modification of the plant, and not its growth, which records the
significant responses to stimuli. For this reason the discussion of adaptation
in the pages that follow is practically confined to modification of structure.
This is particularly desirable, since growth has long been the theme of
physiological study, while modification has too often been considered from
the structural standpoint alone. The comparatively few studies that have
taken function into account have been largely empirical; in them neither
stimulus nor adaptation has received anything approaching adequate
treatment.
148. The measurement of response. The amount of response to a
stimulus is proportional to the intensity of the factor concerned. This does
not mean that the same stimulus produces the same response in two distinct
species, or necessarily in two plants of one species. In these cases the rule
holds only when the plants or species are equally plastic. For each
individual, however, this quantitative correspondence of stimulus and
response is fundamental. It is uncertain whether an exact or constant ratio
can be established between factor and function; the answer to this must
await the general use of quantitative methods. There can be no doubt,
however, that within certain limits the adjustment is proportional to the
amount of stimulus, whereas reaction is well known to be abnormal or
inhibited beyond certain extremes. It is quite erroneous to think that
reaction is independent of quantity of stimulus, or to liken the stimulating
factor to “the smallest spark (which) by igniting a mass of powder, produces
an enormous mechanical effect.”[8] Such a statement is only apparently true
of the action of mechanical stimuli upon the few plants that may properly
be said to possess irritability, such as sensitive plants and certain
insectivorous ones. Of the normal relation of response to direct factors,
water, light, etc., it is entirely untrue. Axiomatically, there is ordinarily an
essential correspondence, also, between the amount of adjustment and of
adaptation. This correspondence is profoundly affected, however, by the
structural stability of the plant.
Page 145
From the preceding it follows that the measurement of response and the
relating it to definite amounts of direct factors as stimuli are two of the most
fundamental tasks of ecology. The exact determination of physical factors
has no value apart from its use for this purpose. It is perfectly clear that
precise methods of measuring stimuli call for similar methods in
determining the amount of adjustment and of adaptation. The problem is a
difficult one, and it is possible at present only to indicate the direction
which its development should take, and to describe a few methods which
will at least serve as a beginning. To cover the ground adequately it is
necessary to measure response by adjustment and by adaptation separately,
and in the latter to find a measure for the individual and one for the species.
The one is furnished by the methods of morphology and the other by
biometry.
A primary requisite for any method for measuring adjustment is that it be
applicable to field conditions. Many instruments for measuring
transpiration, for example, are valueless, not because they are inaccurate,
but because the plant studied is under abnormal conditions. To avoid the
latter is absolutely necessary, a fact which makes it peculiarly difficult to
devise a satisfactory field method. After the latter has been found and
applied, it becomes possible to check other methods by it, and to give them
real value. The final test of a field method is three-fold: (1) the plant must
be studied while functioning normally in its own habitat; (2) the method
must give accurate results; and (3) it must permit of extensive and fairly
convenient application in the field. Until methods of this character, some of
which are described later, have been employed for some time, it is
impossible to connect definite intensities of factor stimuli with measured
amounts of adjustment. Ultimately, it seems certain that researches will
regularly take this form.
Adaptation is primarily indicated by changes in the arrangement and
character of the cells of the plant. Since these determine the form of each
organ, morphology also furnishes important evidence in regard to the
course of adaptation, but form can be connected certainly with adjustment
only through the study of cellular adaptation. In tracing the modifications of
cell and of tissue, the usual methods of histology, viz., sectioning and
drawing, suffice for the individual. It is merely necessary to select plants
and organs which are as nearly typical as can be determined. The question
of quantity becomes paramount, however, since it often gives the clue to
relating it to definite amounts of direct factors as stimuli are two of the most
fundamental tasks of ecology. The exact determination of physical factors
has no value apart from its use for this purpose. It is perfectly clear that
precise methods of measuring stimuli call for similar methods in
determining the amount of adjustment and of adaptation. The problem is a
difficult one, and it is possible at present only to indicate the direction
which its development should take, and to describe a few methods which
will at least serve as a beginning. To cover the ground adequately it is
necessary to measure response by adjustment and by adaptation separately,
and in the latter to find a measure for the individual and one for the species.
The one is furnished by the methods of morphology and the other by
biometry.
A primary requisite for any method for measuring adjustment is that it be
applicable to field conditions. Many instruments for measuring
transpiration, for example, are valueless, not because they are inaccurate,
but because the plant studied is under abnormal conditions. To avoid the
latter is absolutely necessary, a fact which makes it peculiarly difficult to
devise a satisfactory field method. After the latter has been found and
applied, it becomes possible to check other methods by it, and to give them
real value. The final test of a field method is three-fold: (1) the plant must
be studied while functioning normally in its own habitat; (2) the method
must give accurate results; and (3) it must permit of extensive and fairly
convenient application in the field. Until methods of this character, some of
which are described later, have been employed for some time, it is
impossible to connect definite intensities of factor stimuli with measured
amounts of adjustment. Ultimately, it seems certain that researches will
regularly take this form.
Adaptation is primarily indicated by changes in the arrangement and
character of the cells of the plant. Since these determine the form of each
organ, morphology also furnishes important evidence in regard to the
course of adaptation, but form can be connected certainly with adjustment
only through the study of cellular adaptation. In tracing the modifications of
cell and of tissue, the usual methods of histology, viz., sectioning and
drawing, suffice for the individual. It is merely necessary to select plants
and organs which are as nearly typical as can be determined. The question
of quantity becomes paramount, however, since it often gives the clue to
Page 146
qualitative changes, and hence it is imperative that complete and accurate
measurements of cells, tissues, and organs be made. These measurements,
when extended to a sufficiently large number of plants, serve to indicate the
direction of adaptation in the species. They constitute the materials for
determining biometrically the mean of adaptation for the species and the
probable evolution of the latter. In its present development, biometry
contains too much mathematics, and too little biology. This has perhaps
been unavoidable, but it is to be hoped that the future will bring about a
wise sifting of methods, which will make biometry the ready and invaluable
servant of all serious students of experimental evolution. This condition
does not obtain at present, and in consequence it seems unwise to consider
the subject of biometry in this treatise.
149. Plasticity and fixity. As the product of accumulated responses, each
species is characterized by a certain ability or inability to react to stimuli.
Many facts seem to indicate that the degree of stability is connected with
the length of time during which the species is acted upon by the same
stimuli. It seems probable that plants which have reacted to sunlight for
hundreds of years will respond less readily to shade than those which have
grown in the sun for a much shorter period. This hypothesis is not
susceptible of proof in nature because it is ordinarily impossible to
distinguish species upon the basis of the time during which they have
occupied one habitat. Evidence and ultimate proof, perhaps, can be obtained
only by field and control experiments, in which the time of occupation of
any habitat is definitely known. Even in this case, however, it is clear that
antecedent habitats will have left effects which can neither be traced nor
ignored. Additional support is given this view by the fact that extreme
types, both ecological and taxonomic, are the most stable. Intense
xerophytes and hydrophytes are much more fixed than mesophytes, though
the intensity of the stimulus has doubtless as great an influence as its
duration. Composites, labiates, grasses, orchids, etc., are less plastic than
ranals, rosals, etc., but there are many exceptions to the apparent rule that
fixity increases with taxonomic complexity. At present it seems quite
impossible to suggest an explanation of the rule. Recent experiments
indicate that there may be ancestral fixity of function, as well as of
structure. It has been found, for example, that the flowers of certain species
always react normally to the stimuli which produce opening and closing,
while others make extremely erratic response. If further work confirms this
measurements of cells, tissues, and organs be made. These measurements,
when extended to a sufficiently large number of plants, serve to indicate the
direction of adaptation in the species. They constitute the materials for
determining biometrically the mean of adaptation for the species and the
probable evolution of the latter. In its present development, biometry
contains too much mathematics, and too little biology. This has perhaps
been unavoidable, but it is to be hoped that the future will bring about a
wise sifting of methods, which will make biometry the ready and invaluable
servant of all serious students of experimental evolution. This condition
does not obtain at present, and in consequence it seems unwise to consider
the subject of biometry in this treatise.
149. Plasticity and fixity. As the product of accumulated responses, each
species is characterized by a certain ability or inability to react to stimuli.
Many facts seem to indicate that the degree of stability is connected with
the length of time during which the species is acted upon by the same
stimuli. It seems probable that plants which have reacted to sunlight for
hundreds of years will respond less readily to shade than those which have
grown in the sun for a much shorter period. This hypothesis is not
susceptible of proof in nature because it is ordinarily impossible to
distinguish species upon the basis of the time during which they have
occupied one habitat. Evidence and ultimate proof, perhaps, can be obtained
only by field and control experiments, in which the time of occupation of
any habitat is definitely known. Even in this case, however, it is clear that
antecedent habitats will have left effects which can neither be traced nor
ignored. Additional support is given this view by the fact that extreme
types, both ecological and taxonomic, are the most stable. Intense
xerophytes and hydrophytes are much more fixed than mesophytes, though
the intensity of the stimulus has doubtless as great an influence as its
duration. Composites, labiates, grasses, orchids, etc., are less plastic than
ranals, rosals, etc., but there are many exceptions to the apparent rule that
fixity increases with taxonomic complexity. At present it seems quite
impossible to suggest an explanation of the rule. Recent experiments
indicate that there may be ancestral fixity of function, as well as of
structure. It has been found, for example, that the flowers of certain species
always react normally to the stimuli which produce opening and closing,
while others make extremely erratic response. If further work confirms this
Page 147
result and extends it to other functions, the necessity of arriving at a better
understanding of fixity will be greatly emphasized.
It is impossible to make progress in the study of adaptation without
recognizing the fundamental importance of ancestral fixity as a factor. E. S.
Clements[9] has shown that a number of species undergo pronounced
changes in habitat without showing appreciable modification.
Consequently, it is incorrect to assume that each habitat puts a structural
impress upon every plant that enters it. For this reason, the writer feels that
the current explanation of xerophytic bog plants, etc., is probably wrong,
and that the discrepancy between the nature of the habitat and the structure
of the plant is to be explained by the persistence of a fixed ancestral type.
The anomaly is scarcely greater than in cases that have proved capable of
being explained.
150. The law of extremes. When a stimulus approaches either the
maximum or minimum of the factor for the species concerned, response
becomes abnormal. The resulting modifications approach each other and in
some respects at least become similar. Such effects are found chiefly in
growth, but they occur to some degree in structure also. It is imperative that
they be recognized in nature as well as in field and control experiment,
since they directly affect the ratio between response and stimulus. The data
which bear upon the similarity of response to extremes of different factors
are too meager to permit the formulation of a rule. It is permissible,
however, to suggest the general principle that extreme stimuli produce
similar growth responses, and to emphasize the need of testing its
application to adaptation proper.
151. The method of working hypotheses. In the study of stimulus and
response, where the unimpeachable facts are relatively few, and their
present correlation slight, the working hypothesis is an indispensable aid.
“The true course of inductive procedure ... consists in anticipating nature,
in the sense of forming hypotheses as to the laws which are probably in
operation, and then observing whether the combinations of phenomena are
such as would follow from the laws supposed. The investigator begins with
facts and ends with them. He uses such facts as are in the first place known
to him in suggesting probable hypotheses; deducing other facts which
would happen if a particular hypothesis is true, he proceeds to test the truth
of his notion by fresh observations or experiments. If any result prove
understanding of fixity will be greatly emphasized.
It is impossible to make progress in the study of adaptation without
recognizing the fundamental importance of ancestral fixity as a factor. E. S.
Clements[9] has shown that a number of species undergo pronounced
changes in habitat without showing appreciable modification.
Consequently, it is incorrect to assume that each habitat puts a structural
impress upon every plant that enters it. For this reason, the writer feels that
the current explanation of xerophytic bog plants, etc., is probably wrong,
and that the discrepancy between the nature of the habitat and the structure
of the plant is to be explained by the persistence of a fixed ancestral type.
The anomaly is scarcely greater than in cases that have proved capable of
being explained.
150. The law of extremes. When a stimulus approaches either the
maximum or minimum of the factor for the species concerned, response
becomes abnormal. The resulting modifications approach each other and in
some respects at least become similar. Such effects are found chiefly in
growth, but they occur to some degree in structure also. It is imperative that
they be recognized in nature as well as in field and control experiment,
since they directly affect the ratio between response and stimulus. The data
which bear upon the similarity of response to extremes of different factors
are too meager to permit the formulation of a rule. It is permissible,
however, to suggest the general principle that extreme stimuli produce
similar growth responses, and to emphasize the need of testing its
application to adaptation proper.
151. The method of working hypotheses. In the study of stimulus and
response, where the unimpeachable facts are relatively few, and their
present correlation slight, the working hypothesis is an indispensable aid.
“The true course of inductive procedure ... consists in anticipating nature,
in the sense of forming hypotheses as to the laws which are probably in
operation, and then observing whether the combinations of phenomena are
such as would follow from the laws supposed. The investigator begins with
facts and ends with them. He uses such facts as are in the first place known
to him in suggesting probable hypotheses; deducing other facts which
would happen if a particular hypothesis is true, he proceeds to test the truth
of his notion by fresh observations or experiments. If any result prove
Page 148
different from what he expects, it leads him either to abandon or to modify
his hypothesis; but every new fact may give some new suggestion as to the
laws in action. Even if the result in any case agrees with his anticipations,
he does not regard it as finally confirmatory of his theory, but proceeds to
test the truth of the theory by new deductions and new trials.”[10] In the
treatment of adjustment and adaptation which follows, the method of
multiple working hypotheses is uniformly employed. No apology is felt to
be necessary for this, since the whole endeavor is to indicate the proper
points of attack, and not to distinguish between that which is conjectural
and that which is known. If an hypothesis occasionally seem to be stated
too strongly, it is merely that it appears, after a survey of the problem from
all sides, to explain the facts most satisfactorily. The final proof of any
hypothesis, however, rests not only upon its ability to explain all the facts,
but also upon the inability of other hypotheses to meet the same test. The
discovery and examination of all possible hypotheses, and the elimination
of those that prove inadequate are the essential steps in the method of
working hypotheses.
his hypothesis; but every new fact may give some new suggestion as to the
laws in action. Even if the result in any case agrees with his anticipations,
he does not regard it as finally confirmatory of his theory, but proceeds to
test the truth of the theory by new deductions and new trials.”[10] In the
treatment of adjustment and adaptation which follows, the method of
multiple working hypotheses is uniformly employed. No apology is felt to
be necessary for this, since the whole endeavor is to indicate the proper
points of attack, and not to distinguish between that which is conjectural
and that which is known. If an hypothesis occasionally seem to be stated
too strongly, it is merely that it appears, after a survey of the problem from
all sides, to explain the facts most satisfactorily. The final proof of any
hypothesis, however, rests not only upon its ability to explain all the facts,
but also upon the inability of other hypotheses to meet the same test. The
discovery and examination of all possible hypotheses, and the elimination
of those that prove inadequate are the essential steps in the method of
working hypotheses.
Page 149
HYDROHARMOSE
ADJUSTMENT
152. Water as a stimulus. Plants are continually subjected to the action
of the water of the soil and of the air; exception must naturally be made of
submerged plants. The stimulus of soil water acts upon the absorbing organ,
the root, while that of humidity affects the part most exposed to the air, viz.,
the assimilative organ, which is normally the leaf. But since both are
simultaneous water stimuli, a clearer conception is gained of this operation
if they are viewed as two phases of the same stimulus. This point of view
receives further warrant from the essential and intimate relation of humidity
and water-content as determined by the plant. They are in fact largely
compensatory, as is shown at some length later. In determining the intensity
of the two, a significant difference between them must be recognized. The
total humidity of the air at any one time constitutes a stimulus to the leaf
which it touches. This is not true of the total soil water. Part of the latter is
not available under any circumstances, and can not affect the plant, at least
directly. The chresard alone can act as a stimulus, but even this is potential
in the great majority of cases, since the actual stimulus is not the water
available but the water absorbed. The latter, moreover, contains many
nutrient salts which are in themselves stimuli, but as they normally have
little bearing upon the action of water as a stimulus they are to be
considered only when present in excessive amounts.
153. The influence of other factors upon water. The amount of
humidity is modified directly by temperature, wind, precipitation, and
pressure, and, through these, it is affected by altitude, slope, exposure, and
cover. Naturally, also, the evaporation of soil water has a marked influence.
In determining water-content, atmospheric factors, with the exception of
precipitation, are usually subordinate to edaphic ones. Soil texture, slope,
and precipitation act directly in determining soil water, while temperature,
wind, and pressure can operate only through humidity. This is likewise true
of altitude, exposure, and cover, though the latter has in addition a profound
effect upon run-off. Biotic factors can affect humidity or water-content only
ADJUSTMENT
152. Water as a stimulus. Plants are continually subjected to the action
of the water of the soil and of the air; exception must naturally be made of
submerged plants. The stimulus of soil water acts upon the absorbing organ,
the root, while that of humidity affects the part most exposed to the air, viz.,
the assimilative organ, which is normally the leaf. But since both are
simultaneous water stimuli, a clearer conception is gained of this operation
if they are viewed as two phases of the same stimulus. This point of view
receives further warrant from the essential and intimate relation of humidity
and water-content as determined by the plant. They are in fact largely
compensatory, as is shown at some length later. In determining the intensity
of the two, a significant difference between them must be recognized. The
total humidity of the air at any one time constitutes a stimulus to the leaf
which it touches. This is not true of the total soil water. Part of the latter is
not available under any circumstances, and can not affect the plant, at least
directly. The chresard alone can act as a stimulus, but even this is potential
in the great majority of cases, since the actual stimulus is not the water
available but the water absorbed. The latter, moreover, contains many
nutrient salts which are in themselves stimuli, but as they normally have
little bearing upon the action of water as a stimulus they are to be
considered only when present in excessive amounts.
153. The influence of other factors upon water. The amount of
humidity is modified directly by temperature, wind, precipitation, and
pressure, and, through these, it is affected by altitude, slope, exposure, and
cover. Naturally, also, the evaporation of soil water has a marked influence.
In determining water-content, atmospheric factors, with the exception of
precipitation, are usually subordinate to edaphic ones. Soil texture, slope,
and precipitation act directly in determining soil water, while temperature,
wind, and pressure can operate only through humidity. This is likewise true
of altitude, exposure, and cover, though the latter has in addition a profound
effect upon run-off. Biotic factors can affect humidity or water-content only
Page 150
through the medium of another factor. Light in itself has no action upon
either, but through its conversion into heat within the chloroplast, it has a
profound effect upon transpiration. The following table indicates the
general relation between water and the other physical factors of the habitat.
The order of the signs, ±, denotes that the water increases and decreases
with an increase and decrease of the factor, or the reverse, ∓.
Humidity ± Water-content ±
Temperature ∓ Temperature ∓
Wind ∓ Wind ∓
Precipitation ± Precipitation ±
Pressure ± Pressure ∓
Soil texture 0 Soil texture
Altitude ∓ Porosity ∓
Capillarity ±
Slope ∓ Slope ∓
Exposure ∓ Exposure ∓
Cover ± Cover ±
154. Response. The normal functional responses to water stimuli are
absorption, diffusion, transport, and transpiration. Of these, absorption and
transpiration alone are the immediate response to soil water and humidity,
respectively. Consequently they are the critical points of attack in studying
the fundamental relation of the plant to the water of its habitat. In
determining the pathway of the response, it is necessary to trace the steps in
diffusion and transport, but, as these are essentially alike for all vascular
plants, this task lies outside the scope of the work in hand. As previously
suggested, the relation between absorption and transpiration is strictly
compensatory, though, for obvious reasons, the amount of water transpired
is usually somewhat less than the amount absorbed. Absorption falls below
transpiration when extreme conditions cause temporary or permanent
wilting; the two activities are essentially equal after a growing plant reaches
maturity. In all cases, however, the rule is that an increase or decrease in
water loss produces a corresponding change in the amount of water
absorbed, and, conversely, variation in absorption produces a consequent
change in transpiration. This is strictly true only when the stimuli are
normal. For example, a decrease in humidity causes increased water loss,
which, through diffusion and transport, is compensated by increased
either, but through its conversion into heat within the chloroplast, it has a
profound effect upon transpiration. The following table indicates the
general relation between water and the other physical factors of the habitat.
The order of the signs, ±, denotes that the water increases and decreases
with an increase and decrease of the factor, or the reverse, ∓.
Humidity ± Water-content ±
Temperature ∓ Temperature ∓
Wind ∓ Wind ∓
Precipitation ± Precipitation ±
Pressure ± Pressure ∓
Soil texture 0 Soil texture
Altitude ∓ Porosity ∓
Capillarity ±
Slope ∓ Slope ∓
Exposure ∓ Exposure ∓
Cover ± Cover ±
154. Response. The normal functional responses to water stimuli are
absorption, diffusion, transport, and transpiration. Of these, absorption and
transpiration alone are the immediate response to soil water and humidity,
respectively. Consequently they are the critical points of attack in studying
the fundamental relation of the plant to the water of its habitat. In
determining the pathway of the response, it is necessary to trace the steps in
diffusion and transport, but, as these are essentially alike for all vascular
plants, this task lies outside the scope of the work in hand. As previously
suggested, the relation between absorption and transpiration is strictly
compensatory, though, for obvious reasons, the amount of water transpired
is usually somewhat less than the amount absorbed. Absorption falls below
transpiration when extreme conditions cause temporary or permanent
wilting; the two activities are essentially equal after a growing plant reaches
maturity. In all cases, however, the rule is that an increase or decrease in
water loss produces a corresponding change in the amount of water
absorbed, and, conversely, variation in absorption produces a consequent
change in transpiration. This is strictly true only when the stimuli are
normal. For example, a decrease in humidity causes increased water loss,
which, through diffusion and transport, is compensated by increased
Page 151
activity of the root surface. Frequently the water supply is insufficient to
compensate for a greater stimulus, and the proper balance can be attained
only by the closing of the stomata. In the case of excessive stimuli, neither
compensation suffices, and the plant dies. Many mesophytes and all
xerophytes have probably resulted from stimuli which regularly approached
the limit of compensation for each, and often overstepped, but never
permanently exceeded it. For hydrophytes, the danger arises from excessive
water supply, not water loss. There is a limit to the compensation afforded
by transpiration, which is naturally dependent upon the amount of plant
surface exposed to the air. No compensation occurs in the case of
submerged plants; floating hydrophytes possess a single transpiring leaf
surface, while the leaves of amphibious plants behave as do those of
mesophytes. The whole question of response to water stimuli thus turns
upon the compensation for water loss afforded by water supply where the
latter is moderate or precarious, and upon the compensation for water
supply furnished by water loss where the supply is excessive, submerged
plants excepted.
155. The measurement of absorption. As responses to measured stimuli
of water-content and humidity, it is imperative that the amount of
absorption and of transpiration be determined quantitatively. It is also
extremely desirable that this be done in the normal habitat of the plant. A
careful examination of the problems to be met quickly discloses the great
difficulty of obtaining a direct and accurate measure of absorption under
normal conditions, especially in the field. For this purpose, the ordinary
potometric experiments by means of cut stems are valueless. The use of the
entire plant in a potometer yields much more trustworthy results, though the
fact that the root is under abnormal conditions can not be overlooked,
especially in the case of mesophytes and xerophytes. While potometric
conditions are less abnormal for amphibious plants, the error is not wholly
eliminated, since the roots normally grow in the soil. The potometer can be
made of value for quantitative work only by checking the results it gives by
means of an instrument or a method in which the plant functions normally.
In consequence, the potometer can not at present be used to measure
absorption directly, though, as is further indicated in the discussion of
transpiration, it is a valuable supplementary instrument, after the check
mentioned has been applied to its use with a particular species.
compensate for a greater stimulus, and the proper balance can be attained
only by the closing of the stomata. In the case of excessive stimuli, neither
compensation suffices, and the plant dies. Many mesophytes and all
xerophytes have probably resulted from stimuli which regularly approached
the limit of compensation for each, and often overstepped, but never
permanently exceeded it. For hydrophytes, the danger arises from excessive
water supply, not water loss. There is a limit to the compensation afforded
by transpiration, which is naturally dependent upon the amount of plant
surface exposed to the air. No compensation occurs in the case of
submerged plants; floating hydrophytes possess a single transpiring leaf
surface, while the leaves of amphibious plants behave as do those of
mesophytes. The whole question of response to water stimuli thus turns
upon the compensation for water loss afforded by water supply where the
latter is moderate or precarious, and upon the compensation for water
supply furnished by water loss where the supply is excessive, submerged
plants excepted.
155. The measurement of absorption. As responses to measured stimuli
of water-content and humidity, it is imperative that the amount of
absorption and of transpiration be determined quantitatively. It is also
extremely desirable that this be done in the normal habitat of the plant. A
careful examination of the problems to be met quickly discloses the great
difficulty of obtaining a direct and accurate measure of absorption under
normal conditions, especially in the field. For this purpose, the ordinary
potometric experiments by means of cut stems are valueless. The use of the
entire plant in a potometer yields much more trustworthy results, though the
fact that the root is under abnormal conditions can not be overlooked,
especially in the case of mesophytes and xerophytes. While potometric
conditions are less abnormal for amphibious plants, the error is not wholly
eliminated, since the roots normally grow in the soil. The potometer can be
made of value for quantitative work only by checking the results it gives by
means of an instrument or a method in which the plant functions normally.
In consequence, the potometer can not at present be used to measure
absorption directly, though, as is further indicated in the discussion of
transpiration, it is a valuable supplementary instrument, after the check
mentioned has been applied to its use with a particular species.
Page 152
An estimate of the amount of absorption may be obtained either in the
field or in the control house by taking samples from the protected soil at
different times. Since it is impossible to determine the weight of the area in
which the roots lie, and since the soil water is often unequally distributed,
this method can not yield exact results. An accurate method of measuring
absorption under essentially normal conditions has been devised and tested
in the control house. The essential feature of the process is the placing a
plant in a soil containing a known quantity of water, and removing it after it
has absorbed water from the soil for a certain period. In carrying out the
experiment, a soil consisting of two parts of sod and one of sand was used,
since the aeration is more perfect and the particles are more easily removed
from the roots. The soil was completely dried out in a water bath and then
placed in a five-inch battery jar. The latter, together with the rubber cloth
used later to prevent evaporation, was weighed to the decigram. A weighed
quantity of water was added, and the whole again weighed as a check. Two
plants of Helianthus annuus were taken from the pots in which they had
grown, and the soil was carefully washed from the roots. Each plant was
weighed with its roots in a dish of water to prevent wilting, and then
carefully potted, one in each battery jar. A thistle tube was placed in the soil
of each jar to facilitate aeration, as well as the addition of weighed amounts
of water, when necessary, and the rubber cloth attached in the usual manner
to prevent evaporation. The entire outfit was weighed again, and the
weighing repeated at 8:00 A.M. and 5:00 P.M. for five days, in order to
determine the amount of transpiration and its relation to the water absorbed.
The plants were kept in diffuse light to prevent excessive water loss while
the roots were becoming established. At the close of the experiment, the jar
and its contents were weighed finally. The plants were removed and
weighed, the soil particles being shaken from the roots into the jar, which
was also weighed. The results obtained were as follows:
Wt. of pot and Wt. of pot and wet Total H2O H 2O H2O
dry soil soil H 2O left absorbed transpired
I II
I 1846.0 g. 2218.0 2174.3 372.0 g. 328.3 43.7 g. 43.7 g.
g. g. g.
II 1886.7 g. 2253.2 2221.6 366.5 g. 334.9 31.6 g. 31.6 g.
g. g. g.
The amount of water absorbed may be obtained directly by subtracting the
final weight of the jar and moist soil from their first weight, but a desirable
field or in the control house by taking samples from the protected soil at
different times. Since it is impossible to determine the weight of the area in
which the roots lie, and since the soil water is often unequally distributed,
this method can not yield exact results. An accurate method of measuring
absorption under essentially normal conditions has been devised and tested
in the control house. The essential feature of the process is the placing a
plant in a soil containing a known quantity of water, and removing it after it
has absorbed water from the soil for a certain period. In carrying out the
experiment, a soil consisting of two parts of sod and one of sand was used,
since the aeration is more perfect and the particles are more easily removed
from the roots. The soil was completely dried out in a water bath and then
placed in a five-inch battery jar. The latter, together with the rubber cloth
used later to prevent evaporation, was weighed to the decigram. A weighed
quantity of water was added, and the whole again weighed as a check. Two
plants of Helianthus annuus were taken from the pots in which they had
grown, and the soil was carefully washed from the roots. Each plant was
weighed with its roots in a dish of water to prevent wilting, and then
carefully potted, one in each battery jar. A thistle tube was placed in the soil
of each jar to facilitate aeration, as well as the addition of weighed amounts
of water, when necessary, and the rubber cloth attached in the usual manner
to prevent evaporation. The entire outfit was weighed again, and the
weighing repeated at 8:00 A.M. and 5:00 P.M. for five days, in order to
determine the amount of transpiration and its relation to the water absorbed.
The plants were kept in diffuse light to prevent excessive water loss while
the roots were becoming established. At the close of the experiment, the jar
and its contents were weighed finally. The plants were removed and
weighed, the soil particles being shaken from the roots into the jar, which
was also weighed. The results obtained were as follows:
Wt. of pot and Wt. of pot and wet Total H2O H 2O H2O
dry soil soil H 2O left absorbed transpired
I II
I 1846.0 g. 2218.0 2174.3 372.0 g. 328.3 43.7 g. 43.7 g.
g. g. g.
II 1886.7 g. 2253.2 2221.6 366.5 g. 334.9 31.6 g. 31.6 g.
g. g. g.
The amount of water absorbed may be obtained directly by subtracting the
final weight of the jar and moist soil from their first weight, but a desirable
Page 153
check is obtained by taking the dry weight of jar and soil from the first, and
the final weight of these, and subtracting the one from the other as indicated
in the table. A second check is afforded by daily weighings, from which the
amount of water transpired is determined. Since the two sunflower plants
made practically no growth during the period of experiment, the exact
correspondence between water absorbed and water lost is not startling,
though it can not be expected that the results will always coincide.
This method has certain slight sources of error, all of which, it is thought,
have been corrected in a new and more complete series of experiments now
being carried on. The aeration of the soil is not entirely normal, as is also
true of the capillary movements of the water, on account of the nonporous
glass jar and the rubber cloth. Since the latter are necessary conditions of all
accurate methods for measuring absorption and transpiration, the resulting
error must be ignored. It can be reduced, however, by forcing air through
the thistle tube from time to time. Sturdy plants, such as the sunflower, are
the most satisfactory, since they recover more quickly from the shock of
transplanting. Almost any plant can be used, however, if repotted in a loose
sandy soil often enough. This permits the root system to develop normally,
and also makes it possible to wash the soil away without injury to the root.
The method is so recent that there has been no opportunity to test it in the
field. It would seem that it can be applied without essential change to plants
in their normal habitats. Very large herbs or plants with extensive root
systems could not be used to advantage, and to be practicable the
experiments would need to be carried on near the base station. The great
value of the method, however, lies in its use as a check in determining the
accuracy of other methods, and in practice it will often be found convenient
and time-saving to use the latter, after they have once been carefully
checked for different groups of species. This matter is further considered
under measures of transpiration.
the final weight of these, and subtracting the one from the other as indicated
in the table. A second check is afforded by daily weighings, from which the
amount of water transpired is determined. Since the two sunflower plants
made practically no growth during the period of experiment, the exact
correspondence between water absorbed and water lost is not startling,
though it can not be expected that the results will always coincide.
This method has certain slight sources of error, all of which, it is thought,
have been corrected in a new and more complete series of experiments now
being carried on. The aeration of the soil is not entirely normal, as is also
true of the capillary movements of the water, on account of the nonporous
glass jar and the rubber cloth. Since the latter are necessary conditions of all
accurate methods for measuring absorption and transpiration, the resulting
error must be ignored. It can be reduced, however, by forcing air through
the thistle tube from time to time. Sturdy plants, such as the sunflower, are
the most satisfactory, since they recover more quickly from the shock of
transplanting. Almost any plant can be used, however, if repotted in a loose
sandy soil often enough. This permits the root system to develop normally,
and also makes it possible to wash the soil away without injury to the root.
The method is so recent that there has been no opportunity to test it in the
field. It would seem that it can be applied without essential change to plants
in their normal habitats. Very large herbs or plants with extensive root
systems could not be used to advantage, and to be practicable the
experiments would need to be carried on near the base station. The great
value of the method, however, lies in its use as a check in determining the
accuracy of other methods, and in practice it will often be found convenient
and time-saving to use the latter, after they have once been carefully
checked for different groups of species. This matter is further considered
under measures of transpiration.
Page 154
Fig. 31. Absorption and transpiration of Helianthus annuus. I and II, plants repotted in soil of known
weight and water-content; III, plant undisturbed in the original soil; IV, potometer containing plant
with cut stem; V, potometer with entire plant.
156. The quantitative relation of absorption and transpiration.
Burgerstein[11] has summarized the results of various investigators in the
statement “that between the quantitative absorption of water on the one
hand and emission on the other there exists no constant parallelism or
proportion,” and he has cited the work of Kröber, and of Eberdt in proof.
This statement holds, however, only for short periods of a few hours, or
more rarely, a day, and even here its truth still remains to be conclusively
demonstrated. The discrepancy between absorption and transpiration for a
short period is often greater than for a longer time, but it is evident that a
transient change in behavior or a small error in the method would inevitably
produce this result. Eberdt found the discrepancy for a few hours to be 1–2
ccm. in an entire plant of Helianthus annuus, while for a whole day the
water absorbed was 33.57 ccm. and the water lost 33.98 ccm. Kröber’s
weight and water-content; III, plant undisturbed in the original soil; IV, potometer containing plant
with cut stem; V, potometer with entire plant.
156. The quantitative relation of absorption and transpiration.
Burgerstein[11] has summarized the results of various investigators in the
statement “that between the quantitative absorption of water on the one
hand and emission on the other there exists no constant parallelism or
proportion,” and he has cited the work of Kröber, and of Eberdt in proof.
This statement holds, however, only for short periods of a few hours, or
more rarely, a day, and even here its truth still remains to be conclusively
demonstrated. The discrepancy between absorption and transpiration for a
short period is often greater than for a longer time, but it is evident that a
transient change in behavior or a small error in the method would inevitably
produce this result. Eberdt found the discrepancy for a few hours to be 1–2
ccm. in an entire plant of Helianthus annuus, while for a whole day the
water absorbed was 33.57 ccm. and the water lost 33.98 ccm. Kröber’s
Page 155
experiments with cut branches of Asclepias incarnata showed a maximum
difference for 12 hours of 2.5 ccm., but the discrepancy for the first 24
hours was 1 ccm. and for the second 1.9 ccm. In both cases, the potometer
was employed. Consequently, as will be shown later, Eberdt’s results are
not entirely trustworthy, while those of Kröber, made with cut stems, are
altogether unreliable. Hence, it is clear that the discrepancy is slight for a
period of several days or weeks, and that it may be ignored without serious
error, except in a few plants that retain considerable water as cell-sap, in
consequence of extremely rapid growth. Accordingly, the amount of
transpiration, which may be readily and accurately determined, can be
employed as a measure of absorption that is sufficiently accurate for nearly
all purposes. The truth of this statement may be easily confirmed. It is
evident that the amount of water absorbed equals the amount transpired plus
that retained by the plant as cell-sap, or used in the manufacture of organic
compounds. In plants not actively growing, the amount lost equals that
absorbed, as already shown in the experiment with Helianthus. According
to Gain[12], Dehérain has found that a plant rooted in ordinary soil transpired
680 kg. of water for each kilogram of dry substance elaborated. In
Helianthus annuus, the dry matter is 10 per cent of the weight of the green
plant. A well-grown plant weighing 1,000 grams, therefore, consists of 100
grams of dry matter and 900 of water. The length of the growing period for
such a plant is approximately 100 days, during which it transpires 68
kilograms of water. Assuming the rate of transpiration and of growth to be
constant, the plant transpires 680 grams daily, adds 9 grams to its cell-sap,
and 1 gram to its dry weight. The amount of water in a gram of cellulose
and its isomers is about ⅗. Consequently, the total water absorbed daily by
the plant is 689.6 grams. The 680 grams transpired are 98.6 per cent of the
amount absorbed; in other words, only 1.4 per cent of the water absorbed is
retained by the plant. From this it is evident that the simplest and most
convenient measure of absorption under normal conditions can be obtained
through transpiration, since the discrepancy between absorption and
transpiration is scarcely larger than the error of any method applicable to
the field. Conversely, the measure of absorption obtained by the process
described in the preceding section serves also as a measure of transpiration.
The determination of the latter in the field is so much simpler, however, that
it is rarely desirable to apply the absorption method.
difference for 12 hours of 2.5 ccm., but the discrepancy for the first 24
hours was 1 ccm. and for the second 1.9 ccm. In both cases, the potometer
was employed. Consequently, as will be shown later, Eberdt’s results are
not entirely trustworthy, while those of Kröber, made with cut stems, are
altogether unreliable. Hence, it is clear that the discrepancy is slight for a
period of several days or weeks, and that it may be ignored without serious
error, except in a few plants that retain considerable water as cell-sap, in
consequence of extremely rapid growth. Accordingly, the amount of
transpiration, which may be readily and accurately determined, can be
employed as a measure of absorption that is sufficiently accurate for nearly
all purposes. The truth of this statement may be easily confirmed. It is
evident that the amount of water absorbed equals the amount transpired plus
that retained by the plant as cell-sap, or used in the manufacture of organic
compounds. In plants not actively growing, the amount lost equals that
absorbed, as already shown in the experiment with Helianthus. According
to Gain[12], Dehérain has found that a plant rooted in ordinary soil transpired
680 kg. of water for each kilogram of dry substance elaborated. In
Helianthus annuus, the dry matter is 10 per cent of the weight of the green
plant. A well-grown plant weighing 1,000 grams, therefore, consists of 100
grams of dry matter and 900 of water. The length of the growing period for
such a plant is approximately 100 days, during which it transpires 68
kilograms of water. Assuming the rate of transpiration and of growth to be
constant, the plant transpires 680 grams daily, adds 9 grams to its cell-sap,
and 1 gram to its dry weight. The amount of water in a gram of cellulose
and its isomers is about ⅗. Consequently, the total water absorbed daily by
the plant is 689.6 grams. The 680 grams transpired are 98.6 per cent of the
amount absorbed; in other words, only 1.4 per cent of the water absorbed is
retained by the plant. From this it is evident that the simplest and most
convenient measure of absorption under normal conditions can be obtained
through transpiration, since the discrepancy between absorption and
transpiration is scarcely larger than the error of any method applicable to
the field. Conversely, the measure of absorption obtained by the process
described in the preceding section serves also as a measure of transpiration.
The determination of the latter in the field is so much simpler, however, that
it is rarely desirable to apply the absorption method.
Page 156
157. Measurement of transpiration. The water loss of a plant may be
determined absolutely or relatively. Absolute or quantitative determinations
are by (1) weighing, (2) collecting, or (3) measuring the water absorbed;
relative values are indicated by hygroscopic substances. A number of
methods have been employed more or less generally for measuring
transpiration. The great majority of these can be used to advantage only in
the laboratory, and practically all fail to meet the fundamental requirement
for successful field work, namely, that the plant be studied under normal
conditions in its own habitat. The following is a summary of the various
methods, the details of which may be found in Burgerstein.
1. Weighing. This is the most satisfactory of all methods for determining
water loss. It is more accurate than any other, and is unique in that it does
not place the plant under abnormal conditions. On the score of convenience,
moreover, it excels every other method capable of yielding quantitative
results. Various modifications of weighing are employed, but none of these
have all the advantages of a direct, simple weighing of the plant in its own
soil.
2. Collecting the water transpired. This may be done by collecting and
weighing the water vapor exhaled by a plant placed within a bell jar, or by
weighing a deliquescent salt, such as calcium chloride, which is used to
absorb the water of transpiration. The decisive disadvantage of these
methods is that transpiration is carried on in an atmosphere far more humid
than normal. If an excessive amount of salt is used, the air is abnormally
dry. In both cases, the water loss decreases until it reaches a point much
below the usual amount. Finally, all methods of this kind are open to
considerable error, and are inconvenient, especially in field work. They are
of relatively slight value in comparison with weighing.
3. Potometers. It has already been shown that the amount of water
absorbed is a close measure of the amount transpired. In consequence, the
potometer can be used to determine the amount of transpiration provided
the absorption is not abnormal. It is rarely and only with much difficulty
that this condition can be met. The use of cut stems and branches does not
meet it, and even in the case of plants with roots, the results must be
compared with those obtained from absorption experiments made with
plants rooted in soil before they can be relied upon. This necessity
practically puts the potometer out of commission for accurate work, unless
determined absolutely or relatively. Absolute or quantitative determinations
are by (1) weighing, (2) collecting, or (3) measuring the water absorbed;
relative values are indicated by hygroscopic substances. A number of
methods have been employed more or less generally for measuring
transpiration. The great majority of these can be used to advantage only in
the laboratory, and practically all fail to meet the fundamental requirement
for successful field work, namely, that the plant be studied under normal
conditions in its own habitat. The following is a summary of the various
methods, the details of which may be found in Burgerstein.
1. Weighing. This is the most satisfactory of all methods for determining
water loss. It is more accurate than any other, and is unique in that it does
not place the plant under abnormal conditions. On the score of convenience,
moreover, it excels every other method capable of yielding quantitative
results. Various modifications of weighing are employed, but none of these
have all the advantages of a direct, simple weighing of the plant in its own
soil.
2. Collecting the water transpired. This may be done by collecting and
weighing the water vapor exhaled by a plant placed within a bell jar, or by
weighing a deliquescent salt, such as calcium chloride, which is used to
absorb the water of transpiration. The decisive disadvantage of these
methods is that transpiration is carried on in an atmosphere far more humid
than normal. If an excessive amount of salt is used, the air is abnormally
dry. In both cases, the water loss decreases until it reaches a point much
below the usual amount. Finally, all methods of this kind are open to
considerable error, and are inconvenient, especially in field work. They are
of relatively slight value in comparison with weighing.
3. Potometers. It has already been shown that the amount of water
absorbed is a close measure of the amount transpired. In consequence, the
potometer can be used to determine the amount of transpiration provided
the absorption is not abnormal. It is rarely and only with much difficulty
that this condition can be met. The use of cut stems and branches does not
meet it, and even in the case of plants with roots, the results must be
compared with those obtained from absorption experiments made with
plants rooted in soil before they can be relied upon. This necessity
practically puts the potometer out of commission for accurate work, unless
Page 157
future study may show a somewhat constant ratio between the absorption of
a plant in its own soil and that of a plant placed in a potometer.
4. Measuring absolute humidity. The cog psychrometer makes it possible
to determine the increased relative humidity produced within a glass
cylinder or special tin chamber by a transpiring plant. From this result the
absolute humidity is readily obtained, and by means of the latter the actual
amount of water given off. The evident drawback to this method is that the
increasing humidity within the chamber gives results entirely abnormal for
the plant concerned.
5. Self-registering instruments. There are various methods for registering
the amount of transpiration, based upon weighing, or upon the potometer.
The Richard recording evaporimeter has all the advantages of weighing,
inasmuch as the water loss is measured in this way, and in addition the
amount is recorded upon a revolving drum, obviating the necessity of
repeated attention in case it is desirable to know the exact course of
transpiration. On the other hand, methods which depend upon the
potometer, while graphic, are not sufficiently accurate to be of value.
6. The use of hygroscopic materials. Hygroscopic substances change
their form or color in response to moisture. As they indicate comparative
water loss alone, they are of value chiefly in the study of the stomatic
surfaces of leaves. F. Darwin[13] has used strips of horn, awns of Stipa, and
epidermis of Yucca to construct small hygroscopes for this purpose. In these
instruments the error is large, but as no endeavor is made to obtain exact
results, it is negligible. Filter paper impregnated with a 3–5 per cent
aqueous solution of cobalt chloride is deep blue when dry. If a strip of
cobalt paper is placed upon a leaf and covered with a glass slip it turns
bright rose color, the rapidity of the change affording a clue to the amount
of transpiration.
158. Field methods. The conditions which a satisfactory field method of
measuring transpiration must fulfill have already been discussed; they are
accuracy, simplicity, and normality. These conditions are met only by
weighing the plant in its own soil and habitat. This has been accomplished
by means of the sheet-iron soil box, already described under the
determination of the chresard. The method is merely the familiar one of pot
and balance, slightly modified for field use. The soil block, which contains
the plant to be studied, is cut out, and the metal plates put in position as
a plant in its own soil and that of a plant placed in a potometer.
4. Measuring absolute humidity. The cog psychrometer makes it possible
to determine the increased relative humidity produced within a glass
cylinder or special tin chamber by a transpiring plant. From this result the
absolute humidity is readily obtained, and by means of the latter the actual
amount of water given off. The evident drawback to this method is that the
increasing humidity within the chamber gives results entirely abnormal for
the plant concerned.
5. Self-registering instruments. There are various methods for registering
the amount of transpiration, based upon weighing, or upon the potometer.
The Richard recording evaporimeter has all the advantages of weighing,
inasmuch as the water loss is measured in this way, and in addition the
amount is recorded upon a revolving drum, obviating the necessity of
repeated attention in case it is desirable to know the exact course of
transpiration. On the other hand, methods which depend upon the
potometer, while graphic, are not sufficiently accurate to be of value.
6. The use of hygroscopic materials. Hygroscopic substances change
their form or color in response to moisture. As they indicate comparative
water loss alone, they are of value chiefly in the study of the stomatic
surfaces of leaves. F. Darwin[13] has used strips of horn, awns of Stipa, and
epidermis of Yucca to construct small hygroscopes for this purpose. In these
instruments the error is large, but as no endeavor is made to obtain exact
results, it is negligible. Filter paper impregnated with a 3–5 per cent
aqueous solution of cobalt chloride is deep blue when dry. If a strip of
cobalt paper is placed upon a leaf and covered with a glass slip it turns
bright rose color, the rapidity of the change affording a clue to the amount
of transpiration.
158. Field methods. The conditions which a satisfactory field method of
measuring transpiration must fulfill have already been discussed; they are
accuracy, simplicity, and normality. These conditions are met only by
weighing the plant in its own soil and habitat. This has been accomplished
by means of the sheet-iron soil box, already described under the
determination of the chresard. The method is merely the familiar one of pot
and balance, slightly modified for field use. The soil block, which contains
the plant to be studied, is cut out, and the metal plates put in position as
Page 158
indicated in section 53. Indeed, it is a great saving of time and effort to
determine transpiration and chresard in the same experiment; this is
particularly desirable in view of the close connection between them. In this
event, the soil block must be small enough not to exceed the load of a field
balance. After the block is cut and encased, all the plants are removed,
except the one to be studied. If several individuals of the same species are
present, it is an advantage to leave all of them, since the error arising from
individual variations of water loss may, in this way, be almost completely
eliminated. A sheet of rubber or rubber cloth is carefully tied over the box to
prevent evaporation from the soil. A broad band is passed under the box to
aid in lifting it upon the scales. The latter must be of the platform type, and
should have a capacity as great as consistent with the need for moving it
about in the field. Weighings are made in the usual way, care being taken to
free the surface of the box from soil. The aeration of the soil block is kept
normal by removing the rubber for a few minutes from time to time, or by
forcing air through a thistle tube. Water is also added through the latter,
when it is desired to continue the experiment for a considerable period.
After the study of transpiration is concluded, the rubber cloth is removed,
soil samples taken, and the soil allowed to dry out until the plant becomes
thoroughly wilted. If the box is weighed again, the difference represents the
amount of available water. The per cent of chresard is also obtained in the
usual way by taking samples for ascertaining the echard, and subtracting
this from the holard. Field determinations of water loss yield the most
valuable results when different habitat forms, or ecads, of the same species
are used. There is little profit in comparing the transpiration of a typical sun
plant, such as Touterea multiflora, with that of a shade plant, such as
Washingtonia obtusa. But the simultaneous study of plants like
Chamaenerium angustifolium, Gentiana acuta, Scutellaria brittonii etc.,
which grow in several different habitats, furnishes direct and fundamental
evidence of the course of adjustment and adaptation.
Hesselmann[14], in his study of open woodlands in Sweden, has employed
a method essentially similar to the preceding. Young plants of various
species were transferred to pots in the field, where they were allowed to
grow for several months before a series of weighings was made to
determine the amount of transpiration. Since weighing is the measure used
in each, both methods are equally accurate. The one has a certain advantage
in that the pots are, perhaps, more easily handled, while the other has the
determine transpiration and chresard in the same experiment; this is
particularly desirable in view of the close connection between them. In this
event, the soil block must be small enough not to exceed the load of a field
balance. After the block is cut and encased, all the plants are removed,
except the one to be studied. If several individuals of the same species are
present, it is an advantage to leave all of them, since the error arising from
individual variations of water loss may, in this way, be almost completely
eliminated. A sheet of rubber or rubber cloth is carefully tied over the box to
prevent evaporation from the soil. A broad band is passed under the box to
aid in lifting it upon the scales. The latter must be of the platform type, and
should have a capacity as great as consistent with the need for moving it
about in the field. Weighings are made in the usual way, care being taken to
free the surface of the box from soil. The aeration of the soil block is kept
normal by removing the rubber for a few minutes from time to time, or by
forcing air through a thistle tube. Water is also added through the latter,
when it is desired to continue the experiment for a considerable period.
After the study of transpiration is concluded, the rubber cloth is removed,
soil samples taken, and the soil allowed to dry out until the plant becomes
thoroughly wilted. If the box is weighed again, the difference represents the
amount of available water. The per cent of chresard is also obtained in the
usual way by taking samples for ascertaining the echard, and subtracting
this from the holard. Field determinations of water loss yield the most
valuable results when different habitat forms, or ecads, of the same species
are used. There is little profit in comparing the transpiration of a typical sun
plant, such as Touterea multiflora, with that of a shade plant, such as
Washingtonia obtusa. But the simultaneous study of plants like
Chamaenerium angustifolium, Gentiana acuta, Scutellaria brittonii etc.,
which grow in several different habitats, furnishes direct and fundamental
evidence of the course of adjustment and adaptation.
Hesselmann[14], in his study of open woodlands in Sweden, has employed
a method essentially similar to the preceding. Young plants of various
species were transferred to pots in the field, where they were allowed to
grow for several months before a series of weighings was made to
determine the amount of transpiration. Since weighing is the measure used
in each, both methods are equally accurate. The one has a certain advantage
in that the pots are, perhaps, more easily handled, while the other has the
Page 159
advantage of maintaining the normal relation of soil and roots, a condition
more or less impossible in a pot. In both instances the weighing should be
done in the habitat, which was not the case in Hesselmann’s researches.
The slight value of the potometer, which has had a vogue far beyond its
merits, is indicated by the following table. These results were obtained from
three plants of Helianthus annuus; III was left undisturbed in the pot where
it had been growing, IV was placed in a potometer, after the root had been
cut off, and V was an entire plant placed in a potometer. The amount of
transpiration is indicated in grams per square decimeter of leaf surface. The
plants were kept in diffuse light, except for a period of two hours (8:00 to
10:00 A.M.) on the last day, when they were in full sunshine at a temperature
of 75° F. Plant IV wilted so promptly in the sunshine that it was found
necessary to conclude the experiment in diffuse light.
8 A.M. 5 P.M. 8 A.M. 5 P.M. 8 A.M. 5 P.M. 8 A.M. 10 A.M. 5 P.M. 8 A.M. Total
III 2.9 7.3 2.4 6.0 1.7 1.6 2.0 3.4 2.0 1.8 31.1
IV 4.7 7.2 2.9 2.3 1.0 0.6 0.9 0.5 0.5 0.4 21.0
V 3.7 5.3 3.2 4.8 2.5 1.6 3.0 2.6 1.6 2.6 30.9
The cut plant, IV, lost more water the first day than either of the others,
but the water loss soon decreased, and at the end of the period was almost
nil. The total transpiration for III and V is much the same, but the range of
variation for periods of 12 hours is from +2 to –1 gram. This experiment is
taken as a fair warrant that the use of cut stems in potometers can not give
accurate results. It is inconclusive, however, as to the merits of potometric
values obtained by means of the entire plant, and further studies are now
being made with reference to this point.
159. Expression of results. From the previous discussion of the relation
between them, it follows that an expression of the amount of transpiration
likewise constitutes an expression of absorption. It is very desirable also
that the latter be based upon root surface and chresard, but the difficulty of
determining the former accurately and readily is at present too great to
make such a basis practicable. In expressing transpiration in exact terms,
the fact that plants of the same species or form are somewhat individual in
their behavior must be constantly reckoned with. In consequence,
experiments should be made upon two or three individuals whenever
possible, in order to avoid the error arising from this source.
more or less impossible in a pot. In both instances the weighing should be
done in the habitat, which was not the case in Hesselmann’s researches.
The slight value of the potometer, which has had a vogue far beyond its
merits, is indicated by the following table. These results were obtained from
three plants of Helianthus annuus; III was left undisturbed in the pot where
it had been growing, IV was placed in a potometer, after the root had been
cut off, and V was an entire plant placed in a potometer. The amount of
transpiration is indicated in grams per square decimeter of leaf surface. The
plants were kept in diffuse light, except for a period of two hours (8:00 to
10:00 A.M.) on the last day, when they were in full sunshine at a temperature
of 75° F. Plant IV wilted so promptly in the sunshine that it was found
necessary to conclude the experiment in diffuse light.
8 A.M. 5 P.M. 8 A.M. 5 P.M. 8 A.M. 5 P.M. 8 A.M. 10 A.M. 5 P.M. 8 A.M. Total
III 2.9 7.3 2.4 6.0 1.7 1.6 2.0 3.4 2.0 1.8 31.1
IV 4.7 7.2 2.9 2.3 1.0 0.6 0.9 0.5 0.5 0.4 21.0
V 3.7 5.3 3.2 4.8 2.5 1.6 3.0 2.6 1.6 2.6 30.9
The cut plant, IV, lost more water the first day than either of the others,
but the water loss soon decreased, and at the end of the period was almost
nil. The total transpiration for III and V is much the same, but the range of
variation for periods of 12 hours is from +2 to –1 gram. This experiment is
taken as a fair warrant that the use of cut stems in potometers can not give
accurate results. It is inconclusive, however, as to the merits of potometric
values obtained by means of the entire plant, and further studies are now
being made with reference to this point.
159. Expression of results. From the previous discussion of the relation
between them, it follows that an expression of the amount of transpiration
likewise constitutes an expression of absorption. It is very desirable also
that the latter be based upon root surface and chresard, but the difficulty of
determining the former accurately and readily is at present too great to
make such a basis practicable. In expressing transpiration in exact terms,
the fact that plants of the same species or form are somewhat individual in
their behavior must be constantly reckoned with. In consequence,
experiments should be made upon two or three individuals whenever
possible, in order to avoid the error arising from this source.
Page 160
Water loss may be expressed either in terms of transpiring surface or of
dry weight. Since there is no constant relation between surface and weight,
the terms are not interchangeable or comparable, and in practice it is
necessary to use one to the exclusion of the other. Obviously, surface
furnishes by far the best basis, on account of its intimate connection with
stomata and air-spaces, a conclusion which Burgerstein (l. c., p. 6) has
shown by experiment to be true. For the best results, the whole transpiring
surface should be determined. This is especially necessary in making
comparisons of different species. In those studies which are of the greatest
value, viz., ecads of the same species, it is scarcely desirable to measure
stem and petiole surfaces, unless these organs show unusual modification.
The actual transpiring surface is constituted by the walls of the cells
bordering the intercellular spaces, but, since it is impossible to determine
the aggregate area of these, or the humidity of the air-spaces themselves, the
leaf surface must be taken as a basis. Since the transpiration through the
stomata is much greater than that through the epidermal walls, the number
of stomata must be taken into account. Since they are usually less abundant
on the upper surface, their number should be determined for both sides of
the leaf. The errors arising from more or less irregular distribution are
eliminated by making counts near the tip, base, and middle of two or three
mature leaves. The most convenient unit of leaf surface is the square
decimeter. The simplest way to determine the total leaf area of a plant is to
outline the leaves upon a homogeneous paper, or to print them upon a
photographic paper. The outlines are then cut out and weighed, and the leaf
area obtained in square decimeters by dividing the total weight by the
weight of a square decimeter of the paper used. The area may also be
readily determined by means of a planimeter.
160. Coefficient of transpiration. At present it does not seem feasible to
express the transpiration of a plant in the form of a definite coefficient, but
it is probable that the application of exact methods to each part of the
problem will finally bring about this result. Meanwhile the following
formula is suggested as a step toward this goal: t = g(u/l)LHT, in which t,
the transpiration relation of a plant, is expressed by the number of grams of
water lost per hour, on a day of sunshine, by one square decimeter of leaf,
considered with reference to the stomata of the two surfaces, and the
amount of the controlling physical factors, light, humidity, and temperature,
at the time of determination. For Helianthus annuus, this formula would
dry weight. Since there is no constant relation between surface and weight,
the terms are not interchangeable or comparable, and in practice it is
necessary to use one to the exclusion of the other. Obviously, surface
furnishes by far the best basis, on account of its intimate connection with
stomata and air-spaces, a conclusion which Burgerstein (l. c., p. 6) has
shown by experiment to be true. For the best results, the whole transpiring
surface should be determined. This is especially necessary in making
comparisons of different species. In those studies which are of the greatest
value, viz., ecads of the same species, it is scarcely desirable to measure
stem and petiole surfaces, unless these organs show unusual modification.
The actual transpiring surface is constituted by the walls of the cells
bordering the intercellular spaces, but, since it is impossible to determine
the aggregate area of these, or the humidity of the air-spaces themselves, the
leaf surface must be taken as a basis. Since the transpiration through the
stomata is much greater than that through the epidermal walls, the number
of stomata must be taken into account. Since they are usually less abundant
on the upper surface, their number should be determined for both sides of
the leaf. The errors arising from more or less irregular distribution are
eliminated by making counts near the tip, base, and middle of two or three
mature leaves. The most convenient unit of leaf surface is the square
decimeter. The simplest way to determine the total leaf area of a plant is to
outline the leaves upon a homogeneous paper, or to print them upon a
photographic paper. The outlines are then cut out and weighed, and the leaf
area obtained in square decimeters by dividing the total weight by the
weight of a square decimeter of the paper used. The area may also be
readily determined by means of a planimeter.
160. Coefficient of transpiration. At present it does not seem feasible to
express the transpiration of a plant in the form of a definite coefficient, but
it is probable that the application of exact methods to each part of the
problem will finally bring about this result. Meanwhile the following
formula is suggested as a step toward this goal: t = g(u/l)LHT, in which t,
the transpiration relation of a plant, is expressed by the number of grams of
water lost per hour, on a day of sunshine, by one square decimeter of leaf,
considered with reference to the stomata of the two surfaces, and the
amount of the controlling physical factors, light, humidity, and temperature,
at the time of determination. For Helianthus annuus, this formula would
Page 161
appear as follows: t = 2(²⁰⁰⁄₂₅₀) : 1 : 50 : 75°. To avoid the large figures
arising from the extent of surface considered, the number of stomata per
square decimeter is divided by 10,000. This amounts to the number per
square millimeter, and time may consequently be saved by using this figure
directly. While this formula obviously leaves much to be desired, it has the
great advantage of making it possible to compare ecads of one species, or
species of the same habitat or of different habitats, upon an exact basis of
factor, function, and structure.
ADAPTATION
161. Modifications due to water stimuli. In adaptation, the great
desideratum is to connect each modification quantitatively with the
corresponding adjustment. This is even more difficult than to ascertain the
quantitative relation between stimulus and functional response, a task still
beset with serious obstacles. At the present time, little more can be done
than to indicate the relation of marked adaptations of organs and tissues to
the direct factors operating upon them, and to attempt to point out among
the functions possibly concerned the one which seems to be the most
probable connection between the probable stimulus and the structure under
investigation. In the pages that follow, no more than this is attempted. The
general changes of organs and tissues produced by water are first discussed,
and after this is given a summary of the structural features of the plant types
based upon water-content.
162. Modifications due to a small water supply. A water supply which
may become deficient at any time is compensated either by changes which
decrease transpiration, or by those that increase the amount of water
absorbed or stored. These operate upon the form and size of the organs
concerned, as well as upon their structure. Modifications of the form of leaf
and stem are alike in that they lessen transpiration by a reduction of the
amount of surface exposed to the air. Structural adaptations, on the other
hand, bring about the protection of epidermal cells and stomata, and often
internal cells also, from the factors which cause transpiration, or they
anticipate periods of excessive transpiration by the storage of water in
specialized cells or tissues. In certain extreme types the epidermis is itself
modified for the absorption of water vapor from the air.
arising from the extent of surface considered, the number of stomata per
square decimeter is divided by 10,000. This amounts to the number per
square millimeter, and time may consequently be saved by using this figure
directly. While this formula obviously leaves much to be desired, it has the
great advantage of making it possible to compare ecads of one species, or
species of the same habitat or of different habitats, upon an exact basis of
factor, function, and structure.
ADAPTATION
161. Modifications due to water stimuli. In adaptation, the great
desideratum is to connect each modification quantitatively with the
corresponding adjustment. This is even more difficult than to ascertain the
quantitative relation between stimulus and functional response, a task still
beset with serious obstacles. At the present time, little more can be done
than to indicate the relation of marked adaptations of organs and tissues to
the direct factors operating upon them, and to attempt to point out among
the functions possibly concerned the one which seems to be the most
probable connection between the probable stimulus and the structure under
investigation. In the pages that follow, no more than this is attempted. The
general changes of organs and tissues produced by water are first discussed,
and after this is given a summary of the structural features of the plant types
based upon water-content.
162. Modifications due to a small water supply. A water supply which
may become deficient at any time is compensated either by changes which
decrease transpiration, or by those that increase the amount of water
absorbed or stored. These operate upon the form and size of the organs
concerned, as well as upon their structure. Modifications of the form of leaf
and stem are alike in that they lessen transpiration by a reduction of the
amount of surface exposed to the air. Structural adaptations, on the other
hand, bring about the protection of epidermal cells and stomata, and often
internal cells also, from the factors which cause transpiration, or they
anticipate periods of excessive transpiration by the storage of water in
specialized cells or tissues. In certain extreme types the epidermis is itself
modified for the absorption of water vapor from the air.
Page 162
163. The decrease of water loss. The following is a summary of the
contrivances for reducing transpiration.
1. Position of the leaf. Since the energy of a ray of sunlight is greatest at
the sun’s highest altitudes, those leaves transpire least which are in such a
position during midday that the rays strike them as obliquely as possible. A
leaf at right angles to the noonday sun receives ten times as much light and
heat upon a square decimeter of surface as does one placed at an angle of 10
degrees. This device for reducing the intensity of insolation is best
developed in the erect or hanging leaves of many tropical trees. In
temperate zones, it is found in such plants as Silphium laciniatum and
Lactuca scariola, and in species with equitant leaves. In such plants as
Helianthus annuus, the effect is just the opposite, since the turning of the
crown keeps the leaves for a long time at a high angle to the incident rays.
In the case of mats, it is the aggregation of plants which brings about the
mutual protection of the leaves from insolation and wind.
2. Rolling of the leaf. Many grasses and ericaceous plants possess leaves
capable of rolling or folding themselves together when drouth threatens. In
other cases, the leaves are permanently rolled or folded. The advantage of
this device arises not only from the reduction of surface, but also from the
fact that the stomata come to lie in a chamber more or less completely
closed. In the case of those mosses whose leaves roll or twist, a reduction of
surface alone is effected.
3. Reduction of leaf. The transpiring surface of a plant is reduced by
decreasing the number of leaves, by reducing the size of each leaf, or by a
change in its form. In so far as the stem is a leaf, a decrease in size or a
change in shape brings about the same result. The final outcome of
reduction in size or number is the complete loss of leaves, and more rarely,
of the stem. Such marked decrease of leaf area is found only in intense
xerophytes, though it occurs in all deciduous trees as a temporary
adaptation. Changes in leaf form are nearly always accompanied by a
decrease in size. Of the forms which result, the scale, the linear or
cylindrical leaf, and the succulent leaf are the most common. Leaves which
show a tendency to divide often increase the number of lobes or make them
smaller.
4. Epidermal modifications. Excretions of wax and lime by the epidermis
have a pronounced effect by increasing the impermeability of the cuticle,
contrivances for reducing transpiration.
1. Position of the leaf. Since the energy of a ray of sunlight is greatest at
the sun’s highest altitudes, those leaves transpire least which are in such a
position during midday that the rays strike them as obliquely as possible. A
leaf at right angles to the noonday sun receives ten times as much light and
heat upon a square decimeter of surface as does one placed at an angle of 10
degrees. This device for reducing the intensity of insolation is best
developed in the erect or hanging leaves of many tropical trees. In
temperate zones, it is found in such plants as Silphium laciniatum and
Lactuca scariola, and in species with equitant leaves. In such plants as
Helianthus annuus, the effect is just the opposite, since the turning of the
crown keeps the leaves for a long time at a high angle to the incident rays.
In the case of mats, it is the aggregation of plants which brings about the
mutual protection of the leaves from insolation and wind.
2. Rolling of the leaf. Many grasses and ericaceous plants possess leaves
capable of rolling or folding themselves together when drouth threatens. In
other cases, the leaves are permanently rolled or folded. The advantage of
this device arises not only from the reduction of surface, but also from the
fact that the stomata come to lie in a chamber more or less completely
closed. In the case of those mosses whose leaves roll or twist, a reduction of
surface alone is effected.
3. Reduction of leaf. The transpiring surface of a plant is reduced by
decreasing the number of leaves, by reducing the size of each leaf, or by a
change in its form. In so far as the stem is a leaf, a decrease in size or a
change in shape brings about the same result. The final outcome of
reduction in size or number is the complete loss of leaves, and more rarely,
of the stem. Such marked decrease of leaf area is found only in intense
xerophytes, though it occurs in all deciduous trees as a temporary
adaptation. Changes in leaf form are nearly always accompanied by a
decrease in size. Of the forms which result, the scale, the linear or
cylindrical leaf, and the succulent leaf are the most common. Leaves which
show a tendency to divide often increase the number of lobes or make them
smaller.
4. Epidermal modifications. Excretions of wax and lime by the epidermis
have a pronounced effect by increasing the impermeability of the cuticle,
Page 163
and, hence, decreasing epidermal transpiration. It seems improbable that a
coating of wax on the lower surface of a diphotic leaf can have this purpose.
The thickening of the outer wall of epidermal cells to form a cuticle is the
most perfect of all contrivances for decreasing permeability and reducing
transpiration. In many desert plants, the greatly thickened cuticle effectually
prevents epidermal transpiration. In these also the cuticle is regularly
developed in such a way as to protect the guard cells, and even to close the
opening partially. An epidermis consisting of two or more layers of cells is
an effective, though less frequent device against water loss. When
combined with a cuticle, as is usually the case, the impermeability is almost
complete. Hairs decrease transpiration by screening the epidermis so that
the amount of light and heat is diminished, and the access and movement of
dry air impeded. While hairs assume the most various forms, all hairy
coverings serve the same purpose, even when, as in the case of
Mesembryanthemum, they are primarily for water-storage. Hairs protect
stomata as well as epidermal cells: the greater number of the former on the
lower surface readily explains the occurrence of a hairy covering on this
surface, even though absent on the more exposed upper side. In some cases,
hairs are developed only where they serve to screen the stomata.
The modifications of the stomata with respect to transpiration are
numerous, yet all may be classed with reference to changes of number or
level. With the exception of aquatic and some shade plants, the number of
stomata is normally greater on the less exposed, i. e., lower surface. The
number on both surfaces decreases regularly as the danger of excessive
water loss increases, but the decrease is usually more rapid on the upper
surface, which finally loses its stomata entirely. It has been shown by many
observers that species growing in dry places have fewer stomata to the same
area than do those found in moist habitats. This result has been verified
experimentally by the writer in the case of Ranunculus sceleratus, in which,
however, the upper surface possesses the larger number of stomata. Plants
of this species, which normally grow on wet banks, were grown in water so
that the leaves floated, and in soils containing approximately 10, 15, 30, and
40 per cent of water. The averages for the respective forms were: upper 20,
lower 0; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8; upper
10, lower 6. Reduction of number is effective, however, only under
moderate conditions of dryness. As the latter becomes intense, the guard
cells are sunken below the epidermis, either singly or in groups. In both
coating of wax on the lower surface of a diphotic leaf can have this purpose.
The thickening of the outer wall of epidermal cells to form a cuticle is the
most perfect of all contrivances for decreasing permeability and reducing
transpiration. In many desert plants, the greatly thickened cuticle effectually
prevents epidermal transpiration. In these also the cuticle is regularly
developed in such a way as to protect the guard cells, and even to close the
opening partially. An epidermis consisting of two or more layers of cells is
an effective, though less frequent device against water loss. When
combined with a cuticle, as is usually the case, the impermeability is almost
complete. Hairs decrease transpiration by screening the epidermis so that
the amount of light and heat is diminished, and the access and movement of
dry air impeded. While hairs assume the most various forms, all hairy
coverings serve the same purpose, even when, as in the case of
Mesembryanthemum, they are primarily for water-storage. Hairs protect
stomata as well as epidermal cells: the greater number of the former on the
lower surface readily explains the occurrence of a hairy covering on this
surface, even though absent on the more exposed upper side. In some cases,
hairs are developed only where they serve to screen the stomata.
The modifications of the stomata with respect to transpiration are
numerous, yet all may be classed with reference to changes of number or
level. With the exception of aquatic and some shade plants, the number of
stomata is normally greater on the less exposed, i. e., lower surface. The
number on both surfaces decreases regularly as the danger of excessive
water loss increases, but the decrease is usually more rapid on the upper
surface, which finally loses its stomata entirely. It has been shown by many
observers that species growing in dry places have fewer stomata to the same
area than do those found in moist habitats. This result has been verified
experimentally by the writer in the case of Ranunculus sceleratus, in which,
however, the upper surface possesses the larger number of stomata. Plants
of this species, which normally grow on wet banks, were grown in water so
that the leaves floated, and in soils containing approximately 10, 15, 30, and
40 per cent of water. The averages for the respective forms were: upper 20,
lower 0; upper 18, lower 10; upper 18, lower 11; upper 11, lower 8; upper
10, lower 6. Reduction of number is effective, however, only under
moderate conditions of dryness. As the latter becomes intense, the guard
cells are sunken below the epidermis, either singly or in groups. In both
Page 164
cases, the protection is the same, the guard cells and the opening between
them being withdrawn from the intense insolation and the dry air. The sun
rays penetrate the chimney-shaped chambers of sunken stomata only for a
few minutes each day, and they are practically excluded from the stomatal
hollows which are filled with hairs. The influence of dry winds is very
greatly diminished, as is also true, though to a less degree, for leaves in
which the stomata are arranged in furrows. Sunken stomata often have
valve-like projections of cuticle which reduce the opening also. Finally, in a
few plants, water loss in times of drouth is almost completely prevented by
closing the opening with a wax excretion.
5. Modifications in the chlorenchym. A decrease in the size and number
of the air passages in the leaf renders the movement of water-laden air to
the stomata more difficult, and effects a corresponding decrease in
transpiration. The increase of palisade tissue, though primarily dependent
upon light, reduces the air-spaces, and consequently the amount of water
lost. The development of sclereids below the epidermis likewise hinders the
escape of water. Finally the character of the cell sap often plays an
important part, since cells with high salt-content or those containing
mucilaginous substances give up their water with reluctance.
164. The increase of water supply. Plants of dry habitats can increase
their absorption only by modifying the root system so that the absorbing
surfaces are carried into the deep-seated layers of soil, and the surfaces in
contact with the dry soil are protected by means of a cortex. Exception must
be made for epiphytes and a few other plants that absorb rain water and dew
through their leaves, and for those desert plants that seem to condense the
moisture of the air by means of hygroscopic salts, and absorb it through the
epidermis of the leaf. The storage of water in the leaf is a very important
device; it increases the water supply by storing the surplus of absorbed
water against the time of need. Modifications for water-storage are
occasionally found in roots and stems, but their chief development takes
place in the leaf. The epidermis frequently serves as a reservoir for water,
either by the use of the epidermal cells themselves, by the formation of
hypodermal water layers, or by means of superficial bulliform cells. The
water cells of the chlorenchym regularly appear in the form of large clear
cells, scattered singly or arranged in groups. In this event, they occur either
as transverse bands, or as horizontal layers, lying between the palisade and
sponge areas, and connecting the bundles. A few plants possess tracheid-
them being withdrawn from the intense insolation and the dry air. The sun
rays penetrate the chimney-shaped chambers of sunken stomata only for a
few minutes each day, and they are practically excluded from the stomatal
hollows which are filled with hairs. The influence of dry winds is very
greatly diminished, as is also true, though to a less degree, for leaves in
which the stomata are arranged in furrows. Sunken stomata often have
valve-like projections of cuticle which reduce the opening also. Finally, in a
few plants, water loss in times of drouth is almost completely prevented by
closing the opening with a wax excretion.
5. Modifications in the chlorenchym. A decrease in the size and number
of the air passages in the leaf renders the movement of water-laden air to
the stomata more difficult, and effects a corresponding decrease in
transpiration. The increase of palisade tissue, though primarily dependent
upon light, reduces the air-spaces, and consequently the amount of water
lost. The development of sclereids below the epidermis likewise hinders the
escape of water. Finally the character of the cell sap often plays an
important part, since cells with high salt-content or those containing
mucilaginous substances give up their water with reluctance.
164. The increase of water supply. Plants of dry habitats can increase
their absorption only by modifying the root system so that the absorbing
surfaces are carried into the deep-seated layers of soil, and the surfaces in
contact with the dry soil are protected by means of a cortex. Exception must
be made for epiphytes and a few other plants that absorb rain water and dew
through their leaves, and for those desert plants that seem to condense the
moisture of the air by means of hygroscopic salts, and absorb it through the
epidermis of the leaf. The storage of water in the leaf is a very important
device; it increases the water supply by storing the surplus of absorbed
water against the time of need. Modifications for water-storage are
occasionally found in roots and stems, but their chief development takes
place in the leaf. The epidermis frequently serves as a reservoir for water,
either by the use of the epidermal cells themselves, by the formation of
hypodermal water layers, or by means of superficial bulliform cells. The
water cells of the chlorenchym regularly appear in the form of large clear
cells, scattered singly or arranged in groups. In this event, they occur either
as transverse bands, or as horizontal layers, lying between the palisade and
sponge areas, and connecting the bundles. A few plants possess tracheid-
Page 165
like cells which also serve to store water. In the case of succulent leaves,
practically the whole chlorenchym is used for storing water, though they
owe their ability to withstand transpiration to a combination of factors.
165. Modifications due to an excessive water supply. Water plants with
aerial leaf surfaces are modified in such manner as to increase water loss
and to decrease water supply, but the resulting modifications are rarely
striking. There is a marked tendency to increase the exposed surface. This is
indicated by the fact that, while the leaves of mud and floating forms
become larger, they change little or not at all in thickness. The lobing of
leaves is also greatly reduced, or the lobes come to overlap. Leaves of water
plants are practically destitute of all modifications of epidermis and
stomata, which could serve to hinder transpiration. The stomata are usually
more numerous on the upper surface, and in the same species their number
is greater in the forms grown in wet places. These facts explain in part the
extreme development of air-passages in water plants, though this is, in large
measure, a response to the increasing difficulty of aeration. The increase of
air-spaces is correlated with reduction of the palisade, and a decided
increase in the sponge. An increase in water supply is indicated by the
absence of storage tissues, and the reduction of the vascular system, which,
however, is more closely connected with a diminished need for mechanical
support.
166. Plant types. The necessity
for decreasing or increasing water
loss in compensation of the water
supply has made it possible to
distinguish two fundamental
groups of plants upon the twofold
basis of habitat and structure.
These familiar groups, xerophytes
Fig. 32. Mesophyll of Pedicularis and hydrophytes, represent two
procera (chresard, 15%, light, 1). × extremes of habitat and structure,
130. between which lies a more or less
vague, intermediate condition
represented by mesophytes. These
show no characteristic modifications, and it is consequently impossible to
arrange them in subgroups. Xerophytes and hydrophytes, on the other hand,
exhibit marked diversity among themselves, a fact that makes it desirable to
practically the whole chlorenchym is used for storing water, though they
owe their ability to withstand transpiration to a combination of factors.
165. Modifications due to an excessive water supply. Water plants with
aerial leaf surfaces are modified in such manner as to increase water loss
and to decrease water supply, but the resulting modifications are rarely
striking. There is a marked tendency to increase the exposed surface. This is
indicated by the fact that, while the leaves of mud and floating forms
become larger, they change little or not at all in thickness. The lobing of
leaves is also greatly reduced, or the lobes come to overlap. Leaves of water
plants are practically destitute of all modifications of epidermis and
stomata, which could serve to hinder transpiration. The stomata are usually
more numerous on the upper surface, and in the same species their number
is greater in the forms grown in wet places. These facts explain in part the
extreme development of air-passages in water plants, though this is, in large
measure, a response to the increasing difficulty of aeration. The increase of
air-spaces is correlated with reduction of the palisade, and a decided
increase in the sponge. An increase in water supply is indicated by the
absence of storage tissues, and the reduction of the vascular system, which,
however, is more closely connected with a diminished need for mechanical
support.
166. Plant types. The necessity
for decreasing or increasing water
loss in compensation of the water
supply has made it possible to
distinguish two fundamental
groups of plants upon the twofold
basis of habitat and structure.
These familiar groups, xerophytes
Fig. 32. Mesophyll of Pedicularis and hydrophytes, represent two
procera (chresard, 15%, light, 1). × extremes of habitat and structure,
130. between which lies a more or less
vague, intermediate condition
represented by mesophytes. These
show no characteristic modifications, and it is consequently impossible to
arrange them in subgroups. Xerophytes and hydrophytes, on the other hand,
exhibit marked diversity among themselves, a fact that makes it desirable to
Page 166
recognize subgroups, which correspond to fundamental differences of
habitat or adaptation. It is hardly necessary to point out that these types are
not sharply defined, or that a single plastic species may be so modified as to
exhibit several of them. The extremes are always clearly defined, however,
and they indicate the specific tendency of the adaptation shown by other
members of the same group.
167. Xerophytic types. With the exception of dissophytes, all xerophytes
agree in the possession of a deep-seated root system, adapted to withdraw
water from the lower moist layers, and to conserve from loss from the upper
dry layers. Reservoirs are developed in the root, however, in relatively few
cases. The stem follows the leaf more or less closely in its modification,
except when the leaf is greatly reduced or disappears, in which event the
stem exhibits peculiar adaptations. While the leaf is by far the most
strikingly modified, it is a difficult task to employ it satisfactorily as the
basis for distinguishing types. Several adaptations are often combined in the
same leaf, and it is only where one of these is preeminently developed, as in
the case of succulence, that the plant can be referred to a definite type. The
latter does not happen in many species of the less intensely xerophytic
habitats, and, consequently, it is difficult, if not undesirable, to place such
xerophytes under a particular group. The best that can be done is to
recognize the types arising from extreme or characteristic modification, and
to connect the less marked forms as closely as possible with these.
Halophytes differ from xerophytes only in the fact that the chresard is
determined by the salt-content of the habitat, and not by the texture of the
soil. In consequence, they should not be treated as a distinct group.
168. Types of leaf xerophytes. In these, adaptation has acted primarily
upon the leaf, while the stem has remained normal for the most part. Even
when the leaves have become scale-like, they persist throughout the
growing season, and continue to play the primary part in photosynthesis.
The following types may be distinguished:
1. The normal form. The leaf is of the usual dorsiventral character. In
place of a reduction in size, structural modifications are used to decrease
transpiration. With respect to the protective feature that is predominant,
three subtypes may be recognized. The cutinized leaf compensates for a low
water-content by means of a thick cuticle, often reinforced by a high
development of palisade tissue. Such leaves are more or less leathery, and
habitat or adaptation. It is hardly necessary to point out that these types are
not sharply defined, or that a single plastic species may be so modified as to
exhibit several of them. The extremes are always clearly defined, however,
and they indicate the specific tendency of the adaptation shown by other
members of the same group.
167. Xerophytic types. With the exception of dissophytes, all xerophytes
agree in the possession of a deep-seated root system, adapted to withdraw
water from the lower moist layers, and to conserve from loss from the upper
dry layers. Reservoirs are developed in the root, however, in relatively few
cases. The stem follows the leaf more or less closely in its modification,
except when the leaf is greatly reduced or disappears, in which event the
stem exhibits peculiar adaptations. While the leaf is by far the most
strikingly modified, it is a difficult task to employ it satisfactorily as the
basis for distinguishing types. Several adaptations are often combined in the
same leaf, and it is only where one of these is preeminently developed, as in
the case of succulence, that the plant can be referred to a definite type. The
latter does not happen in many species of the less intensely xerophytic
habitats, and, consequently, it is difficult, if not undesirable, to place such
xerophytes under a particular group. The best that can be done is to
recognize the types arising from extreme or characteristic modification, and
to connect the less marked forms as closely as possible with these.
Halophytes differ from xerophytes only in the fact that the chresard is
determined by the salt-content of the habitat, and not by the texture of the
soil. In consequence, they should not be treated as a distinct group.
168. Types of leaf xerophytes. In these, adaptation has acted primarily
upon the leaf, while the stem has remained normal for the most part. Even
when the leaves have become scale-like, they persist throughout the
growing season, and continue to play the primary part in photosynthesis.
The following types may be distinguished:
1. The normal form. The leaf is of the usual dorsiventral character. In
place of a reduction in size, structural modifications are used to decrease
transpiration. With respect to the protective feature that is predominant,
three subtypes may be recognized. The cutinized leaf compensates for a low
water-content by means of a thick cuticle, often reinforced by a high
development of palisade tissue. Such leaves are more or less leathery, and
Page 167
they are often evergreen also.
Arctostaphylus and many species
of Pentstemon are good examples.
Lanate leaves, i. e., those with
dense hairy coverings on one or
both surfaces, as Artemisia,
Antennaria, etc., regularly lack
both cuticle and palisade tissue.
The protection against water loss,
however, is so perfect that the
chlorenchym often assumes the
loose structure of a shade leaf.
Storage leaves usually have a well-
developed cuticle and several rows
of palisade cells, but their
characteristic feature is the water-
storage tissue, which maintains a
reserve supply of water for the
time of extreme drouth. Xerophytic
species of Helianthus furnish
examples of transverse bundles of
storage cells, while those of Fig. 33. Staurophyll of Bahia
Mertensia illustrate the more dissecta, showing extreme
frequent arrangement in which the development of palisade (chresard, 3–
water tissue forms horizontal 9%; light, 1). × 130.
layers.
2. The succulent form. Many succulent leaves are normal in shape and
size, though always thicker than ordinary leaves. Usually, however, they are
reduced in size and are more or less cylindrical in form. The necessary
decrease in transpiration is effected by the reduction in surface, the general
storage of water, a waxy coating, and, often also, by a very thick cuticle.
Agave, Mesembryanthemum, Sedum, and Senecio furnish excellent
examples of this type.
3. The dissected form. The reduction in surface is brought about by the
division of the leaf blade into narrow linear or thread-like lobes which are
widely separated. The latter are themselves protected by a hairy covering or
a thick cuticle, which is often supplemented by many rows of palisade, or
Arctostaphylus and many species
of Pentstemon are good examples.
Lanate leaves, i. e., those with
dense hairy coverings on one or
both surfaces, as Artemisia,
Antennaria, etc., regularly lack
both cuticle and palisade tissue.
The protection against water loss,
however, is so perfect that the
chlorenchym often assumes the
loose structure of a shade leaf.
Storage leaves usually have a well-
developed cuticle and several rows
of palisade cells, but their
characteristic feature is the water-
storage tissue, which maintains a
reserve supply of water for the
time of extreme drouth. Xerophytic
species of Helianthus furnish
examples of transverse bundles of
storage cells, while those of Fig. 33. Staurophyll of Bahia
Mertensia illustrate the more dissecta, showing extreme
frequent arrangement in which the development of palisade (chresard, 3–
water tissue forms horizontal 9%; light, 1). × 130.
layers.
2. The succulent form. Many succulent leaves are normal in shape and
size, though always thicker than ordinary leaves. Usually, however, they are
reduced in size and are more or less cylindrical in form. The necessary
decrease in transpiration is effected by the reduction in surface, the general
storage of water, a waxy coating, and, often also, by a very thick cuticle.
Agave, Mesembryanthemum, Sedum, and Senecio furnish excellent
examples of this type.
3. The dissected form. The reduction in surface is brought about by the
division of the leaf blade into narrow linear or thread-like lobes which are
widely separated. The latter are themselves protected by a hairy covering or
a thick cuticle, which is often supplemented by many rows of palisade, or
Page 168
by storage tissue. Artemisia,
Senecio, and Gilia contain species
which serve as good examples of
this type.
4. The grass form. Xerophytic
grasses and sedges have narrow
filamentous leaves with
longitudinal furrows which serve
to protect the stomata. The furrows
are sometimes filled with hairs
Fig. 34. Diplophyll of Mertensia which are an additional protection,
linearis, showing water cells and the leaves often protect
(chresard, 3–9%, light, 1). × 130. themselves further by rolling up
into a thread-like shape. The
elongated subulate leaves of
Juncus and certain Cyperaceae are essentially of this type, although they
are usually not furrowed.
5. The needle form. This is the typical leaf of conifers, in which a
sweeping reduction of the leaf surface is an absolute necessity. The
relatively small water loss of the needle leaf is still further decreased by a
thick cuticle, and usually also by hypodermal layers of sclerenchyma.
6. The roll form. Roll leaves are frequently small and linear. Their
characteristic feature is produced by the rolling in of the margin on the
under side, by which an almost completely closed chamber is formed for
the protection of the stomata which are regularly confined to the lower
surface of the leaf. The upper epidermis is heavily cutinized and the lower
one often protected by hairs. This type is found especially among the genera
of the Ericales, but it also occurs in a large number of related families.
7. The scale form. Reduction of leaf surface for preventing excessive
water loss reaches its logical culmination in the scale leaf characteristic of
many trees and shrubs, e. g., Cupressus, Tamarix, etc. Scale leaves are
leathery in texture, short and broad, and closely appressed to the stem, as
well as often overlapping.
169. Types of stem xerophytes. In these types the leaves are deciduous
early in the growing period, reduced to functionless scales, or entirely
absent. The functions of the leaf have been assumed by the stem, which
Senecio, and Gilia contain species
which serve as good examples of
this type.
4. The grass form. Xerophytic
grasses and sedges have narrow
filamentous leaves with
longitudinal furrows which serve
to protect the stomata. The furrows
are sometimes filled with hairs
Fig. 34. Diplophyll of Mertensia which are an additional protection,
linearis, showing water cells and the leaves often protect
(chresard, 3–9%, light, 1). × 130. themselves further by rolling up
into a thread-like shape. The
elongated subulate leaves of
Juncus and certain Cyperaceae are essentially of this type, although they
are usually not furrowed.
5. The needle form. This is the typical leaf of conifers, in which a
sweeping reduction of the leaf surface is an absolute necessity. The
relatively small water loss of the needle leaf is still further decreased by a
thick cuticle, and usually also by hypodermal layers of sclerenchyma.
6. The roll form. Roll leaves are frequently small and linear. Their
characteristic feature is produced by the rolling in of the margin on the
under side, by which an almost completely closed chamber is formed for
the protection of the stomata which are regularly confined to the lower
surface of the leaf. The upper epidermis is heavily cutinized and the lower
one often protected by hairs. This type is found especially among the genera
of the Ericales, but it also occurs in a large number of related families.
7. The scale form. Reduction of leaf surface for preventing excessive
water loss reaches its logical culmination in the scale leaf characteristic of
many trees and shrubs, e. g., Cupressus, Tamarix, etc. Scale leaves are
leathery in texture, short and broad, and closely appressed to the stem, as
well as often overlapping.
169. Types of stem xerophytes. In these types the leaves are deciduous
early in the growing period, reduced to functionless scales, or entirely
absent. The functions of the leaf have been assumed by the stem, which
Page 169
exhibits many of the structural adaptations of the former. Warming[15] has
distinguished the following groups:
1. The phyllode form. The petiole is broadened and takes the place of the
leaf blade which is lacking. In other cases, the stem is flattened or winged,
and it replaces the entire leaf. This type occurs in Acacia, Baccharis,
Genista, etc.
2. The virgate form. The leaves either fall off early or they are reduced to
functionless scales. The stems are thin, erect, and rod-like, and are often
greatly branched. They are heavily cutinized and palisaded, and the stomata
are frequently in longitudinal furrows. This type is characteristic of the
Genisteae; it is also found in Ephedra, many species of Polygonum,
Lygodesmia, etc.
3. The rush form. In Heleocharis, many species of Juncus, Scirpus, and
other Cyperaceae, the stem, which is nearly or completely leafless, is
cylindrical and unbranched. It usually possesses also a thick cuticle, and
several rows of dense palisade tissue.
4. The cladophyll form. In Asparagus the leaves are reduced to mere
functionless scales, and their function is assumed by the small needle-
shaped branches.
5. The flattened form. As in the preceding type, the place of the scale-like
leaves is taken by cladophylls, which are more or less flattened and leaf-
like. Ruscus is a familiar illustration of this form.
6. The thorn form. This is typical of many spiny desert shrubs, in which
the leaves are lost very early, or, when present, are mere functionless scales.
The stems have an extremely thick cuticle, and the stomata are deeply
sunken, as a rule. Colletia and Holacantha are good examples of the type.
7. The succulent form. Plants with succulent stems such as the
Cactaceae, Stapelia, and Euphorbia have not only decreased water loss by
extreme reduction or loss of the leaves, and the reduction of stem surface,
but they also offset transpiration by means of storage tissues containing a
mucilaginous sap. The cuticle is usually highly developed and the stomata
sunken. Thorns and spines are also more or less characteristic features.
170. Bog plants. Many of the xerophytic types just described are found
in ponds, bogs, and swamps, where the water supply is excessive, and
hydrophytes would be expected. The explanation that “swamp xerophytes”
distinguished the following groups:
1. The phyllode form. The petiole is broadened and takes the place of the
leaf blade which is lacking. In other cases, the stem is flattened or winged,
and it replaces the entire leaf. This type occurs in Acacia, Baccharis,
Genista, etc.
2. The virgate form. The leaves either fall off early or they are reduced to
functionless scales. The stems are thin, erect, and rod-like, and are often
greatly branched. They are heavily cutinized and palisaded, and the stomata
are frequently in longitudinal furrows. This type is characteristic of the
Genisteae; it is also found in Ephedra, many species of Polygonum,
Lygodesmia, etc.
3. The rush form. In Heleocharis, many species of Juncus, Scirpus, and
other Cyperaceae, the stem, which is nearly or completely leafless, is
cylindrical and unbranched. It usually possesses also a thick cuticle, and
several rows of dense palisade tissue.
4. The cladophyll form. In Asparagus the leaves are reduced to mere
functionless scales, and their function is assumed by the small needle-
shaped branches.
5. The flattened form. As in the preceding type, the place of the scale-like
leaves is taken by cladophylls, which are more or less flattened and leaf-
like. Ruscus is a familiar illustration of this form.
6. The thorn form. This is typical of many spiny desert shrubs, in which
the leaves are lost very early, or, when present, are mere functionless scales.
The stems have an extremely thick cuticle, and the stomata are deeply
sunken, as a rule. Colletia and Holacantha are good examples of the type.
7. The succulent form. Plants with succulent stems such as the
Cactaceae, Stapelia, and Euphorbia have not only decreased water loss by
extreme reduction or loss of the leaves, and the reduction of stem surface,
but they also offset transpiration by means of storage tissues containing a
mucilaginous sap. The cuticle is usually highly developed and the stomata
sunken. Thorns and spines are also more or less characteristic features.
170. Bog plants. Many of the xerophytic types just described are found
in ponds, bogs, and swamps, where the water supply is excessive, and
hydrophytes would be expected. The explanation that “swamp xerophytes”
Page 170
are due to the presence of humic
acids which inhibit absorption and
aeration in the roots has been
generally accepted. As Schimper
has expressed it, bogs and swamps
are “physiologically dry”, i. e., the
available water is small in amount,
in spite of the great total water-
content. Burgerstein (l. c., 142) has
shown, however, that maize plants
transpire, i. e., absorb, three times
as much water in a solution of 0.5
per cent of oxalic acid as they do in
distilled water, and that branches of
Taxus in a solution containing 1 per
cent of tartaric acid absorb more
than twice as much as in distilled
water. Consequently, it seems
improbable that small quantities of
humic acids should decrease
absorption to the extent necessary
for the production of xerophytes in
ponds and bogs. Indeed, in many
ponds and streams, where
Heleocharis, Scirpus, Juncus, etc.,
grow, not a trace of acid is
Fig. 35. Polygonum bistortoides, a discoverable. Furthermore, plants
stable type: 1, mesophyll (chresard, with a characteristic hydrophytic
25%); 2, xerophyll (chresard, 3–5%). structure throughout, such as
× 130. Ranunculus, Caltha, Ludwigia,
Sagittaria, etc., are regularly found
growing alongside of apparent xerophytes. Many of the latter, furthermore,
show a striking contrast in size and vigor of growth in places where they
grow both upon dry gravel banks and in the water, indicating that the
available water-content is much greater in the latter. Finally, many so-called
“swamp xerophytes” possess typically hydrophytic structures, such as air-
passages, diaphragms, etc. In spite of a growing feeling that the xerophytic
acids which inhibit absorption and
aeration in the roots has been
generally accepted. As Schimper
has expressed it, bogs and swamps
are “physiologically dry”, i. e., the
available water is small in amount,
in spite of the great total water-
content. Burgerstein (l. c., 142) has
shown, however, that maize plants
transpire, i. e., absorb, three times
as much water in a solution of 0.5
per cent of oxalic acid as they do in
distilled water, and that branches of
Taxus in a solution containing 1 per
cent of tartaric acid absorb more
than twice as much as in distilled
water. Consequently, it seems
improbable that small quantities of
humic acids should decrease
absorption to the extent necessary
for the production of xerophytes in
ponds and bogs. Indeed, in many
ponds and streams, where
Heleocharis, Scirpus, Juncus, etc.,
grow, not a trace of acid is
Fig. 35. Polygonum bistortoides, a discoverable. Furthermore, plants
stable type: 1, mesophyll (chresard, with a characteristic hydrophytic
25%); 2, xerophyll (chresard, 3–5%). structure throughout, such as
× 130. Ranunculus, Caltha, Ludwigia,
Sagittaria, etc., are regularly found
growing alongside of apparent xerophytes. Many of the latter, furthermore,
show a striking contrast in size and vigor of growth in places where they
grow both upon dry gravel banks and in the water, indicating that the
available water-content is much greater in the latter. Finally, many so-called
“swamp xerophytes” possess typically hydrophytic structures, such as air-
passages, diaphragms, etc. In spite of a growing feeling that the xerophytic
Page 171
features of certain amphibious plants can not be ascribed to a low chresard
in ponds and swamps, a satisfactory explanation of them has been found but
recently. This explanation has come from the work of E. S. Clements
already cited, in which it was found that certain sun plants underwent no
material structural change when grown in the shade, and that the same was
true also of a few species which grew in two or more habitats of very
different water-content. In accordance with this, it is felt that the xerophytic
features found in amphibious plants are due to the persistence of stable
structures, which were developed when these species were growing in
xerophytic situations. When it is called to mind that monocotyledons, and
especially the grasses, sedges, and rushes, are peculiarly stable, it may be
readily understood how certain ancestral characters have persisted in spite
of a striking change of habitat. Such a hypothesis can only be confirmed by
the methods of experimental evolution, and a critical study of this sort is
now under way.
in ponds and swamps, a satisfactory explanation of them has been found but
recently. This explanation has come from the work of E. S. Clements
already cited, in which it was found that certain sun plants underwent no
material structural change when grown in the shade, and that the same was
true also of a few species which grew in two or more habitats of very
different water-content. In accordance with this, it is felt that the xerophytic
features found in amphibious plants are due to the persistence of stable
structures, which were developed when these species were growing in
xerophytic situations. When it is called to mind that monocotyledons, and
especially the grasses, sedges, and rushes, are peculiarly stable, it may be
readily understood how certain ancestral characters have persisted in spite
of a striking change of habitat. Such a hypothesis can only be confirmed by
the methods of experimental evolution, and a critical study of this sort is
now under way.
Page 172
Fig. 36. Hippuris vulgaris: 1, submerged leaf; 2, aerial leaf. × 130.
171. Hydrophytic types. Hydrophytes permit a fairly sharp division into
three groups, based primarily upon the relation of the leaf surface to the two
media, air and water. In submerged plants, the leaves are constantly below
the water; in amphibious ones, they grow normally in the air. Floating
plants have leaves in which the upper surface is in contact with the air, and
the lower in contact with the water. Transpiration is at a maximum in the
amphibious plant; it is reduced by half in the floating type, and is altogether
absent in submerged plants. Aeration reaches a high development in
amphibious and floating forms, but air-passages are normally absent from
171. Hydrophytic types. Hydrophytes permit a fairly sharp division into
three groups, based primarily upon the relation of the leaf surface to the two
media, air and water. In submerged plants, the leaves are constantly below
the water; in amphibious ones, they grow normally in the air. Floating
plants have leaves in which the upper surface is in contact with the air, and
the lower in contact with the water. Transpiration is at a maximum in the
amphibious plant; it is reduced by half in the floating type, and is altogether
absent in submerged plants. Aeration reaches a high development in
amphibious and floating forms, but air-passages are normally absent from
Page 173
submerged forms except as vestiges. Photosynthesis is marked in the
former, but considerably weakened in the latter. The vascular system, which
attains a moderate development in the amphibious type, is considerably
reduced in floating forms, and it is little more than vestigiate in submerged
ones.
1. The amphibious type. Plants
of this type grow in wet soil or in
shallow water. The leaves are
usually large and entire, the stem
well developed, and the roots
numerous and spreading. In the
majority of cases the leaves are
constantly above the water, but in
some species the lower leaves are
often covered, normally, or by a
rise in level, and they take the form
or structure of submerged leaves.
This is illustrated by Callitriche
autumnalis, Hippuris vulgaris,
Ranunculus delphinifolius,
Proserpinaca palustris, Roripa
americana, etc. The epidermis has
a thin cuticle, or none at all, and is
Fig. 37. Floating leaf of Sparganium destitute of hairs. The stomata are
angustifolium. × 130. numerous and usually more
abundant on the upper than on the
lower surface. The palisade tissue
is represented by one or more well-developed rows, but this portion of the
leaf is regularly thinner than that of the sponge part. The latter contains
large air-passages, or, in the majority of cases, numerous air-chambers,
usually provided with diaphragms. The stems are often palisaded, and are
characterized by longitudinal air-chambers crossed by frequent diaphragms,
which extend downward through the roots.
2. The floating type. With respect to form and the structure of the upper
part of the leaf, floating leaves are essentially similar to those of
amphibious plants. They are usually lacquered or coated with wax to
prevent the stoppage of the stomata by water. Stomata, except as vestiges,
former, but considerably weakened in the latter. The vascular system, which
attains a moderate development in the amphibious type, is considerably
reduced in floating forms, and it is little more than vestigiate in submerged
ones.
1. The amphibious type. Plants
of this type grow in wet soil or in
shallow water. The leaves are
usually large and entire, the stem
well developed, and the roots
numerous and spreading. In the
majority of cases the leaves are
constantly above the water, but in
some species the lower leaves are
often covered, normally, or by a
rise in level, and they take the form
or structure of submerged leaves.
This is illustrated by Callitriche
autumnalis, Hippuris vulgaris,
Ranunculus delphinifolius,
Proserpinaca palustris, Roripa
americana, etc. The epidermis has
a thin cuticle, or none at all, and is
Fig. 37. Floating leaf of Sparganium destitute of hairs. The stomata are
angustifolium. × 130. numerous and usually more
abundant on the upper than on the
lower surface. The palisade tissue
is represented by one or more well-developed rows, but this portion of the
leaf is regularly thinner than that of the sponge part. The latter contains
large air-passages, or, in the majority of cases, numerous air-chambers,
usually provided with diaphragms. The stems are often palisaded, and are
characterized by longitudinal air-chambers crossed by frequent diaphragms,
which extend downward through the roots.
2. The floating type. With respect to form and the structure of the upper
part of the leaf, floating leaves are essentially similar to those of
amphibious plants. They are usually lacquered or coated with wax to
prevent the stoppage of the stomata by water. Stomata, except as vestiges,
Page 174
are found only on the upper surface, and the palisade tissue is much less
developed than the sponge, which is uniformly characterized by large air-
chambers. The stems are elongated, the aerating system is enormously
developed, and the supportive tissues are reduced. In the Lemnaceae, the
leaf and the stem are represented by a mere frond or thallus, and the roots
are in the process of disappearance, e. g., Spirodela has several, Lemna one,
and Wolffia none.
3. The submerged type. Both stem and root have been greatly reduced in
submerged plants, owing to the generalization of absorption and the density
of the water. The leaves are greatly reduced in size and thickness, chiefly, it
would seem, for the purpose of insuring readier aeration and great
illumination. The leaf may be ribbon-like, linear, cylindrical, or finely
dissected. Stomata are sometimes present, but they are functionless and
vestigial. A distinction into palisade and sponge tissues, when present, must
also be regarded as a vestige; the chlorenchym is essentially that of a shade
leaf. The air-chambers are much reduced, and sometimes lacking; they
function doubtless as reservoirs for air obtained from the water.
developed than the sponge, which is uniformly characterized by large air-
chambers. The stems are elongated, the aerating system is enormously
developed, and the supportive tissues are reduced. In the Lemnaceae, the
leaf and the stem are represented by a mere frond or thallus, and the roots
are in the process of disappearance, e. g., Spirodela has several, Lemna one,
and Wolffia none.
3. The submerged type. Both stem and root have been greatly reduced in
submerged plants, owing to the generalization of absorption and the density
of the water. The leaves are greatly reduced in size and thickness, chiefly, it
would seem, for the purpose of insuring readier aeration and great
illumination. The leaf may be ribbon-like, linear, cylindrical, or finely
dissected. Stomata are sometimes present, but they are functionless and
vestigial. A distinction into palisade and sponge tissues, when present, must
also be regarded as a vestige; the chlorenchym is essentially that of a shade
leaf. The air-chambers are much reduced, and sometimes lacking; they
function doubtless as reservoirs for air obtained from the water.
Page 175
PHOTOHARMOSE
ADJUSTMENT
172. Light as a stimulus. In nature, light stimuli are determined by
intensity and not by quality. A single exception is afforded by those aquatic
habitats where the depth of water is great, and in consequence of which
certain rays disappear by absorption more quickly than others. In forests
and thickets, where the leaves transmit only the green and yellow rays, it
would appear that the light which reaches the herbaceous layers is deficient
in red and violet rays. The amount of light transmitted by an ordinary sun
leaf is so small, however, that it has no appreciable effect upon the quality
of the light beneath the facies, which is diffuse white light that has passed
between the leaves. Indeed, it is only in the densest forests that distinct
sunflecks do not appear. Coniferous forests, with a light value less than
.005, which suffices only for mosses, lichens, and a few flowering plants,
show frequent sunflecks. This is convincing evidence that the light of such
habitats is normal in quality. It warrants the conclusion that in all habitats
with an intensity capable of supporting vascular plants the light, no matter
how diffuse, is white light. The direction of the light ray is of slight
importance in the field, apart from the difference in intensity which may
result from it. In habitats with diffuse light, the latter comes normally and
constantly from above. Likewise, in sunny situations, direction can have
little influence, since both the direction and the angle of the incident rays
change continually throughout the day, and the position of the leaf itself is
more or less constantly changed by the wind. The influence of duration
upon the character of light stimuli is difficult to determine. There can be no
question that the time during which a stimulus acts has a profound bearing
upon the response that is made to it. In nature the problem is complicated
by the fact that light stimuli are both continuous and periodic. The duration
of sunlight is determined by the periodic return of night as well as by the
irregular occurrence of clouds. Since one is a regular, and the other at least
a normal happening, it is necessary to consider duration only with respect to
the time of actual sunlight on sunny days, except in the case of formations
ADJUSTMENT
172. Light as a stimulus. In nature, light stimuli are determined by
intensity and not by quality. A single exception is afforded by those aquatic
habitats where the depth of water is great, and in consequence of which
certain rays disappear by absorption more quickly than others. In forests
and thickets, where the leaves transmit only the green and yellow rays, it
would appear that the light which reaches the herbaceous layers is deficient
in red and violet rays. The amount of light transmitted by an ordinary sun
leaf is so small, however, that it has no appreciable effect upon the quality
of the light beneath the facies, which is diffuse white light that has passed
between the leaves. Indeed, it is only in the densest forests that distinct
sunflecks do not appear. Coniferous forests, with a light value less than
.005, which suffices only for mosses, lichens, and a few flowering plants,
show frequent sunflecks. This is convincing evidence that the light of such
habitats is normal in quality. It warrants the conclusion that in all habitats
with an intensity capable of supporting vascular plants the light, no matter
how diffuse, is white light. The direction of the light ray is of slight
importance in the field, apart from the difference in intensity which may
result from it. In habitats with diffuse light, the latter comes normally and
constantly from above. Likewise, in sunny situations, direction can have
little influence, since both the direction and the angle of the incident rays
change continually throughout the day, and the position of the leaf itself is
more or less constantly changed by the wind. The influence of duration
upon the character of light stimuli is difficult to determine. There can be no
question that the time during which a stimulus acts has a profound bearing
upon the response that is made to it. In nature the problem is complicated
by the fact that light stimuli are both continuous and periodic. The duration
of sunlight is determined by the periodic return of night as well as by the
irregular occurrence of clouds. Since one is a regular, and the other at least
a normal happening, it is necessary to consider duration only with respect to
the time of actual sunlight on sunny days, except in the case of formations
Page 176
belonging to regions widely different in the amount of normal sunshine, i.
e., the number of cloudy days. In consequence, duration is really a question
of the intensities which succeed each other during the day. The differences
between these have already been shown to fall within the efficient
difference for light, and for this reason the ratio between the light intensity
of a meadow and of a forest is essentially the ratio between the sums of
light intensity for the two habitats, i. e., the duration. The latter is of
importance only where there is a daily alternation between sunshine and
shadow, as at the edge of forest and thicket, in open woodland, etc. In such
places duration determines the actual stimulus by virtue of the sum of
preponderant intensities. The periodicity of daylight is a stimulus to the
guard cells of stomata, but its relation to intensity in this connection is not
clear.
The amount of change in light intensity necessary to constitute an
efficient stimulus seems to depend upon the existing intensity as well as
upon the plant concerned. Apparently, a certain relative decrease is more
efficient for sun plants than for shade plants. At least, many species sooner
or later reach a point where a difference larger than that which has been
efficient no longer produces a structural response. This has been observed
by E. S. Clements (l. c.) in a number of shade ecads. For example, a form of
Galium boreale, which grew with difficulty in a light value of .002, showed
essentially the leaf structure of the form growing in light of .03, while the
form in full sunlight showed a striking difference in the leaf structure. In
considering the light stimuli of habitats, it is unnecessary to discuss the
stimulus of total darkness upon chlorophyllous plants, although this is of
great importance in experimental evolution and in control experiment. The
normal extremes of light intensity, i. e., those within which chlorenchym
can function, are full sunshine represented by 1, and a diffuseness of .002,
though small flowering plants have once or twice been found in an intensity
of .001. The maximum light value, even on high mountains, never exceeds
1 by more than an inconsiderable amount, except for the temporary
concentration due to drops of dew, rain, etc. It seems improbable that the
concentrating effect of epidermal papillae can do much more than
compensate for the reflection and absorption of the epidermis. Experimental
study has shown that the maximum intensity in nature may be increased
several, if not many times, without injurious results and without an
appreciable increase in the photosynthetic response, thus indicating that the
e., the number of cloudy days. In consequence, duration is really a question
of the intensities which succeed each other during the day. The differences
between these have already been shown to fall within the efficient
difference for light, and for this reason the ratio between the light intensity
of a meadow and of a forest is essentially the ratio between the sums of
light intensity for the two habitats, i. e., the duration. The latter is of
importance only where there is a daily alternation between sunshine and
shadow, as at the edge of forest and thicket, in open woodland, etc. In such
places duration determines the actual stimulus by virtue of the sum of
preponderant intensities. The periodicity of daylight is a stimulus to the
guard cells of stomata, but its relation to intensity in this connection is not
clear.
The amount of change in light intensity necessary to constitute an
efficient stimulus seems to depend upon the existing intensity as well as
upon the plant concerned. Apparently, a certain relative decrease is more
efficient for sun plants than for shade plants. At least, many species sooner
or later reach a point where a difference larger than that which has been
efficient no longer produces a structural response. This has been observed
by E. S. Clements (l. c.) in a number of shade ecads. For example, a form of
Galium boreale, which grew with difficulty in a light value of .002, showed
essentially the leaf structure of the form growing in light of .03, while the
form in full sunlight showed a striking difference in the leaf structure. In
considering the light stimuli of habitats, it is unnecessary to discuss the
stimulus of total darkness upon chlorophyllous plants, although this is of
great importance in experimental evolution and in control experiment. The
normal extremes of light intensity, i. e., those within which chlorenchym
can function, are full sunshine represented by 1, and a diffuseness of .002,
though small flowering plants have once or twice been found in an intensity
of .001. The maximum light value, even on high mountains, never exceeds
1 by more than an inconsiderable amount, except for the temporary
concentration due to drops of dew, rain, etc. It seems improbable that the
concentrating effect of epidermal papillae can do much more than
compensate for the reflection and absorption of the epidermis. Experimental
study has shown that the maximum intensity in nature may be increased
several, if not many times, without injurious results and without an
appreciable increase in the photosynthetic response, thus indicating that the
Page 177
efficient difference increases toward the maximum as well as toward the
minimum.
173. The reception of light stimuli. Rays of light are received by the
epidermis, by which they are more or less modified. Part of the light is
reflected by the outer wall or by the cuticle, particularly when these present
a shining surface. Hairs diffract the light rays, and hairy coverings
consequently have a profound influence in determining stimuli. The walls
and contents of epidermal cells furthermore absorb some of the light,
especially when the cell sap is colored. In consequence of these effects, the
amount of light that reaches the chlorenchym is always less than that
incident upon the leaf, and in many plants, the difference is very great.
According to Haberlandt[16], the epidermal cells of some shade plants show
modifications designed to concentrate the light rays. Of such devices, he
distinguishes two types: one in which the outer epidermal wall is arched,
another in which the inner wall is deeply concave. Although there can be no
question of the effect of lens-shaped epidermal cells, their occurrence does
not altogether support Haberlandt’s view. Arched and papillate epidermal
cells are found in sun plants where they are unnecessary for increasing
illumination, to say the least. A large number of shade plants show cells of
this character, but in many the outer wall is practically a plane. Shade forms
of a species usually have the outer wall more arched or papillate, but this is
not always true, and, in a few cases, it is the lower epidermis alone that
shows this feature. Finally, a localization of this function in certain two-
celled papillae, such as Haberlandt indicates for Fittonia verschaffelti, does
not appear to be plausible.
The epidermis merely receives the light; the perception of the stimulus
normally occurs in those cells that contain chloroplasts. The cytoplasm of
the epidermal cells, as well as that of the chlorenchym cells, is sensitive to
light, but the response produced by the latter is hardly discernible in the
absence of plastids, except in those plants which possess streaming
protoplasm. The daily opening and closing of the stomata, which is due to
light, is evidently connected with the presence of chloroplasts in the guard
cells. Naturally, the perception of light and the corresponding response
occur in the epidermis of many shade and submerged plants which have
chloroplasts in the epidermal cells. Such cases merely serve to confirm the
view that the perception of light stimuli is localized in the chloroplast. In
conformity with this view, the initial response to such stimuli must be
minimum.
173. The reception of light stimuli. Rays of light are received by the
epidermis, by which they are more or less modified. Part of the light is
reflected by the outer wall or by the cuticle, particularly when these present
a shining surface. Hairs diffract the light rays, and hairy coverings
consequently have a profound influence in determining stimuli. The walls
and contents of epidermal cells furthermore absorb some of the light,
especially when the cell sap is colored. In consequence of these effects, the
amount of light that reaches the chlorenchym is always less than that
incident upon the leaf, and in many plants, the difference is very great.
According to Haberlandt[16], the epidermal cells of some shade plants show
modifications designed to concentrate the light rays. Of such devices, he
distinguishes two types: one in which the outer epidermal wall is arched,
another in which the inner wall is deeply concave. Although there can be no
question of the effect of lens-shaped epidermal cells, their occurrence does
not altogether support Haberlandt’s view. Arched and papillate epidermal
cells are found in sun plants where they are unnecessary for increasing
illumination, to say the least. A large number of shade plants show cells of
this character, but in many the outer wall is practically a plane. Shade forms
of a species usually have the outer wall more arched or papillate, but this is
not always true, and, in a few cases, it is the lower epidermis alone that
shows this feature. Finally, a localization of this function in certain two-
celled papillae, such as Haberlandt indicates for Fittonia verschaffelti, does
not appear to be plausible.
The epidermis merely receives the light; the perception of the stimulus
normally occurs in those cells that contain chloroplasts. The cytoplasm of
the epidermal cells, as well as that of the chlorenchym cells, is sensitive to
light, but the response produced by the latter is hardly discernible in the
absence of plastids, except in those plants which possess streaming
protoplasm. The daily opening and closing of the stomata, which is due to
light, is evidently connected with the presence of chloroplasts in the guard
cells. Naturally, the perception of light and the corresponding response
occur in the epidermis of many shade and submerged plants which have
chloroplasts in the epidermal cells. Such cases merely serve to confirm the
view that the perception of light stimuli is localized in the chloroplast. In
conformity with this view, the initial response to such stimuli must be
Page 178
sought in the chloroplast, and the explanation of all adaptations due to light
must be found in the adjustment shown by the chloroplasts.
174. Response of the chloroplast. The fundamental response of a plastid
to light is the manufacture of chlorophyll. In the presence of carbon dioxide
and water, leucoplasts invariably make chlorophyll, and chloroplasts
replace that lost by decomposition, in response to the stimulus exerted by
light. The latter is normally the efficient factor, since water is always
present in the living plant, and carbon dioxide absent only locally at most.
Sun plants which possess a distinct cuticle, however, produce leucoplasts,
not chloroplasts, in the epidermal cells, although these are as strongly
illuminated as the guard cells, which contain numerous chloroplasts. This is
evidently explained by the lack of carbon dioxide in the epidermis. This gas
is practically unable to penetrate the compact cuticle, at least in the small
quantity present in the air. The supply obtained through the stomata is first
levied upon by the guard cells and then by the cells of the chlorenchym,
with the result that the carbon dioxide is all used before it can reach the
epidermal cells. This view is also supported by the presence of chloroplasts
along the sides and lower wall of palisade cells, where there is normally a
narrow air-passage, and their absence along the upper wall when this is
closely pressed against the epidermis, as is usually the case. Furthermore,
the leaves of some mesophytes when grown in the sun develop a cuticle and
contain leucoplasts. Under glass and in the humid air of the greenhouse, the
same plants develop epidermal chloroplasts but no cuticle. This is in entire
harmony with the well-known fact that shade plants and submerged plants
often possess chloroplasts in the epidermis. Although growing in different
media, their leaves agree in the absence of a cuticle, and consequent
absorption of gases through the epidermis. The size, shape, number, and
position of the chloroplasts are largely determined by light, though a
number of factors enter in. No accurate studies of changes in size and shape
have yet been made, though casual measurements have indicated that the
chloroplasts in the shade form of certain species are nearly hemispherical,
while those of the sun form are plane. In the same plants, the number of
chloroplasts is strikingly smaller in the shade form, but exact comparisons
are yet to be made. The position and movement of chloroplasts have been
the subject of repeated study, but the factors which control them are still to
be conclusively indicated. Light is clearly the principal cause, although
there are many cases where a marked change in the light intensity fails to
must be found in the adjustment shown by the chloroplasts.
174. Response of the chloroplast. The fundamental response of a plastid
to light is the manufacture of chlorophyll. In the presence of carbon dioxide
and water, leucoplasts invariably make chlorophyll, and chloroplasts
replace that lost by decomposition, in response to the stimulus exerted by
light. The latter is normally the efficient factor, since water is always
present in the living plant, and carbon dioxide absent only locally at most.
Sun plants which possess a distinct cuticle, however, produce leucoplasts,
not chloroplasts, in the epidermal cells, although these are as strongly
illuminated as the guard cells, which contain numerous chloroplasts. This is
evidently explained by the lack of carbon dioxide in the epidermis. This gas
is practically unable to penetrate the compact cuticle, at least in the small
quantity present in the air. The supply obtained through the stomata is first
levied upon by the guard cells and then by the cells of the chlorenchym,
with the result that the carbon dioxide is all used before it can reach the
epidermal cells. This view is also supported by the presence of chloroplasts
along the sides and lower wall of palisade cells, where there is normally a
narrow air-passage, and their absence along the upper wall when this is
closely pressed against the epidermis, as is usually the case. Furthermore,
the leaves of some mesophytes when grown in the sun develop a cuticle and
contain leucoplasts. Under glass and in the humid air of the greenhouse, the
same plants develop epidermal chloroplasts but no cuticle. This is in entire
harmony with the well-known fact that shade plants and submerged plants
often possess chloroplasts in the epidermis. Although growing in different
media, their leaves agree in the absence of a cuticle, and consequent
absorption of gases through the epidermis. The size, shape, number, and
position of the chloroplasts are largely determined by light, though a
number of factors enter in. No accurate studies of changes in size and shape
have yet been made, though casual measurements have indicated that the
chloroplasts in the shade form of certain species are nearly hemispherical,
while those of the sun form are plane. In the same plants, the number of
chloroplasts is strikingly smaller in the shade form, but exact comparisons
are yet to be made. The position and movement of chloroplasts have been
the subject of repeated study, but the factors which control them are still to
be conclusively indicated. Light is clearly the principal cause, although
there are many cases where a marked change in the light intensity fails to
Page 179
call forth any readjustment of the plastids. The position of air-spaces as
reservoirs of carbon dioxide and the movement of crude and elaborated
materials from cell to cell frequently have much to do with this problem.
Finally, it must be constantly kept in mind that the chloroplasts lie in the
cytoplasm, which is in constant contact with a cell wall. Hence, any force
that affects the shape of the cell will have a corresponding influence upon
the position of the chloroplasts. When it is considered that in many leaves
these four factors play some part in determining the arrangement of the
plastids, it is not difficult to understand that anomalies frequently appear.
It may be laid down as a general principle that chloroplasts tend to place
themselves at right angles to rays of diffuse light and parallel to rays of
sunlight. This statement is borne out by an examination of the leaves of
typical sun and shade species, or of sun and shade forms of the same
species. Cells which receive diffuse light, i. e., sponge cells, normally have
their rows of plastids parallel with the leaf surface, while those in full
sunlight place the rows at right angles to the surface. This disposition at
once suggests the generally accepted view that chloroplasts in diffuse light
are placed in such a way as to receive all the light possible, while those in
sunlight are so arranged as to be protected from the intense illumination.
Many facts support this statement with respect to shade leaves, but the need
of protection in the sun leaf is not clearly indicated. The regular occurrence
of normal chloroplasts in the guard cells seems conclusive proof that full
sunlight is not injurious to them. Although the upper wall of the outer row
of palisade cells is usually free from chloroplasts, yet it is not at all
uncommon to find it covered by them. These two conditions are often found
in cells side by side, indicating that the difference is due to the presence of
carbon dioxide and not to light. In certain species of monocotyledons, the
arrangement of the chloroplasts is the same in both halves of the leaf, and
there is no difference between the sun and shade leaves of the same species.
The experimental results obtained with concentrated sunlight, though
otherwise conflicting, seem to show conclusively that full sunlight does not
injure the chloroplasts of sun plants, and that the position of plastids in
palisade cells is not for the purpose of protection. This arrangement, which
is known as apostrophe, is furthermore often found in shade forms of
heliophytes. In typical shade species, and in submerged plants, the
disposition of plastids on the wall parallel with the leaf surface, viz.,
reservoirs of carbon dioxide and the movement of crude and elaborated
materials from cell to cell frequently have much to do with this problem.
Finally, it must be constantly kept in mind that the chloroplasts lie in the
cytoplasm, which is in constant contact with a cell wall. Hence, any force
that affects the shape of the cell will have a corresponding influence upon
the position of the chloroplasts. When it is considered that in many leaves
these four factors play some part in determining the arrangement of the
plastids, it is not difficult to understand that anomalies frequently appear.
It may be laid down as a general principle that chloroplasts tend to place
themselves at right angles to rays of diffuse light and parallel to rays of
sunlight. This statement is borne out by an examination of the leaves of
typical sun and shade species, or of sun and shade forms of the same
species. Cells which receive diffuse light, i. e., sponge cells, normally have
their rows of plastids parallel with the leaf surface, while those in full
sunlight place the rows at right angles to the surface. This disposition at
once suggests the generally accepted view that chloroplasts in diffuse light
are placed in such a way as to receive all the light possible, while those in
sunlight are so arranged as to be protected from the intense illumination.
Many facts support this statement with respect to shade leaves, but the need
of protection in the sun leaf is not clearly indicated. The regular occurrence
of normal chloroplasts in the guard cells seems conclusive proof that full
sunlight is not injurious to them. Although the upper wall of the outer row
of palisade cells is usually free from chloroplasts, yet it is not at all
uncommon to find it covered by them. These two conditions are often found
in cells side by side, indicating that the difference is due to the presence of
carbon dioxide and not to light. In certain species of monocotyledons, the
arrangement of the chloroplasts is the same in both halves of the leaf, and
there is no difference between the sun and shade leaves of the same species.
The experimental results obtained with concentrated sunlight, though
otherwise conflicting, seem to show conclusively that full sunlight does not
injure the chloroplasts of sun plants, and that the position of plastids in
palisade cells is not for the purpose of protection. This arrangement, which
is known as apostrophe, is furthermore often found in shade forms of
heliophytes. In typical shade species, and in submerged plants, the
disposition of plastids on the wall parallel with the leaf surface, viz.,
Page 180
epistrophe, is more regular, but even here there are numerous exceptions to
the rule.
The absorption of the light stimulus by the green plastid results, under
normal conditions, in the immediate production of carbohydrates, which in
the vast majority of cases soon become visible as grains of starch. The
appearance of starch in the chloroplasts of flowering plants is such a regular
response to the action of light that it is regarded as the normal indication of
photosynthetic activity. The mere presence of chlorophyll is not an
indication of the latter, since chlorophyll sometimes persists in light too
diffuse for photosynthesis. The amount of starch formed is directly
connected with the light intensity, and in consequence it affords a basis for
the quantitative estimation of the response to light. Two responses to light
stimuli have a direct effect upon the amount of transpiration. Of the light
energy absorbed by the chloroplast, only 2.5 per cent is used in
photosynthesis, while 95–98 per cent is converted into heat, and brings
about marked increase in transpiration. Furthermore, in normal turgid
plants, the direct action of light, as is well known, opens the stomata in the
morning and closes them at night.
175. Aeration and
translocation. The movements of
gases and of solutions through the
tissues of the leaf are intimately
connected with photosynthesis, and
hence with responses to light
stimuli. Aeration depends primarily
upon the periodic opening of the
stomata, for, while the carbon
dioxide and oxygen of the air are
able to pass through epidermal
walls not highly cutinized, the
amount obtainable in this manner
is altogether inadequate, if not
negligible. The development of
sponge tissue or aerenchym is
intimately connected with the
stomata. The position and amount
of aerenchym and the relative
the rule.
The absorption of the light stimulus by the green plastid results, under
normal conditions, in the immediate production of carbohydrates, which in
the vast majority of cases soon become visible as grains of starch. The
appearance of starch in the chloroplasts of flowering plants is such a regular
response to the action of light that it is regarded as the normal indication of
photosynthetic activity. The mere presence of chlorophyll is not an
indication of the latter, since chlorophyll sometimes persists in light too
diffuse for photosynthesis. The amount of starch formed is directly
connected with the light intensity, and in consequence it affords a basis for
the quantitative estimation of the response to light. Two responses to light
stimuli have a direct effect upon the amount of transpiration. Of the light
energy absorbed by the chloroplast, only 2.5 per cent is used in
photosynthesis, while 95–98 per cent is converted into heat, and brings
about marked increase in transpiration. Furthermore, in normal turgid
plants, the direct action of light, as is well known, opens the stomata in the
morning and closes them at night.
175. Aeration and
translocation. The movements of
gases and of solutions through the
tissues of the leaf are intimately
connected with photosynthesis, and
hence with responses to light
stimuli. Aeration depends primarily
upon the periodic opening of the
stomata, for, while the carbon
dioxide and oxygen of the air are
able to pass through epidermal
walls not highly cutinized, the
amount obtainable in this manner
is altogether inadequate, if not
negligible. The development of
sponge tissue or aerenchym is
intimately connected with the
stomata. The position and amount
of aerenchym and the relative
Page 181
extent of sponge cells and air- Fig. 38. Ecads of Allionia linearis,
spaces are in part determined by showing position of chloroplasts. The
the number and position of the palisade shows apostrophe, the
breathing pores. The disposition of sponge epistrophe: 1, sun leaf
air spaces has much to do with the (chresard, 2–5%, light, 1); 2, shade
arrangement of chloroplasts in both leaf (chresard, 11%; light, .012); 3,
palisade and sponge tissues. Starch shade leaf (chresard, 11%; light,
formation is also dependent upon .003). × 250.
the presence of air spaces, but,
contrary to what would be
expected, it seems to be independent of their size, since sun leaves, which
assimilate much more actively than shade leaves, have the smallest air
spaces. From this fact, it appears that the rapidity of aeration depends very
largely upon the rapidity with which the gases are used. Translocation
likewise affects the arrangement of the chloroplasts and the formation of
starch. According to Haberlandt, it also plays the principal part in
determining the form and arrangement of the palisade cells. Chloroplasts
are regularly absent at those points of contact where the transfer of
materials is made from cell to cell, though this is not invariably true. Since
air passages are necessarily absent where cell walls touch, it is possible that
this disposition of the plastids is likewise due to the lack of aeration.
Translocation is directly connected with the appearance of starch. As long
as all the sugar made by the chloroplasts is transferred, no starch appears,
but when assimilation begins to exceed translocation, the increasing
concentration of the sugar solution results in the production of starch grains.
The latter is normally the case in all flowering plants, with the exception of
those that form sugar or oil, but no starch. The constant action of
translocation is practically indispensable to starch formation, since an over-
accumulation of carbohydrates decreases assimilation, and finally inhibits it
altogether. In consequence, translocation occurs throughout the day and
night, and by this means the accumulated carbohydrates of one day are
largely or entirely removed before the next.
176. The measurement of responses to light. Responses, such as the
periodic opening and closing of stomata, which are practically the same for
all leaves, are naturally not susceptible of measurement. This is also true of
the transpiration produced by light, but the difficulty in this case is due to
the impossibility of distinguishing between the water loss due to light and
spaces are in part determined by showing position of chloroplasts. The
the number and position of the palisade shows apostrophe, the
breathing pores. The disposition of sponge epistrophe: 1, sun leaf
air spaces has much to do with the (chresard, 2–5%, light, 1); 2, shade
arrangement of chloroplasts in both leaf (chresard, 11%; light, .012); 3,
palisade and sponge tissues. Starch shade leaf (chresard, 11%; light,
formation is also dependent upon .003). × 250.
the presence of air spaces, but,
contrary to what would be
expected, it seems to be independent of their size, since sun leaves, which
assimilate much more actively than shade leaves, have the smallest air
spaces. From this fact, it appears that the rapidity of aeration depends very
largely upon the rapidity with which the gases are used. Translocation
likewise affects the arrangement of the chloroplasts and the formation of
starch. According to Haberlandt, it also plays the principal part in
determining the form and arrangement of the palisade cells. Chloroplasts
are regularly absent at those points of contact where the transfer of
materials is made from cell to cell, though this is not invariably true. Since
air passages are necessarily absent where cell walls touch, it is possible that
this disposition of the plastids is likewise due to the lack of aeration.
Translocation is directly connected with the appearance of starch. As long
as all the sugar made by the chloroplasts is transferred, no starch appears,
but when assimilation begins to exceed translocation, the increasing
concentration of the sugar solution results in the production of starch grains.
The latter is normally the case in all flowering plants, with the exception of
those that form sugar or oil, but no starch. The constant action of
translocation is practically indispensable to starch formation, since an over-
accumulation of carbohydrates decreases assimilation, and finally inhibits it
altogether. In consequence, translocation occurs throughout the day and
night, and by this means the accumulated carbohydrates of one day are
largely or entirely removed before the next.
176. The measurement of responses to light. Responses, such as the
periodic opening and closing of stomata, which are practically the same for
all leaves, are naturally not susceptible of measurement. This is also true of
the transpiration produced by light, but the difficulty in this case is due to
the impossibility of distinguishing between the water loss due to light and
Page 182
that caused by humidity and other
factors. If it were possible to
determine the amount of
chlorophyll or glucose produced,
these could be used as satisfactory
measures of response. As it is, they
can only be determined
approximately by counting the
chloroplasts or starch grains. The
arrangement of the chloroplasts
can not furnish the measure sought,
since it does not lend itself to
quantitative methods, and since the
relation to light intensity is too
inconstant. Hesselmann (l. c., 400)
has determined the amount of
carbon dioxide respired, by means
of a eudiometer, and has based
comparisons of sun and shade
plants upon the results. As he
points out, however, light has no
direct connection with respiration.
Although the latter increases
necessarily with increased
Fig. 39. Position of chloroplasts in nutrition, the relation between
aerial leaf (1) and submerged leaf (2) them is so obscure, and so far from
of Callitriche bifida. × 250. exact, that the amount of
respiration can in no wise serve as
a measure of the response to light.
As a result of the foregoing, it is clear that no functional response is able to
furnish a satisfactory measure of adjustment to light, though one or two
have perhaps sufficient value to warrant their use. Indeed, structural
adaptations offer a much better basis for the quantitative determination of
the effects of light stimuli, as will be shown later.
In attempting to use the number of chloroplasts or starch grains as a
measure of response, the study should be confined to the sun and shade
forms of the same species, or, in some cases, to the forms of closely related
factors. If it were possible to
determine the amount of
chlorophyll or glucose produced,
these could be used as satisfactory
measures of response. As it is, they
can only be determined
approximately by counting the
chloroplasts or starch grains. The
arrangement of the chloroplasts
can not furnish the measure sought,
since it does not lend itself to
quantitative methods, and since the
relation to light intensity is too
inconstant. Hesselmann (l. c., 400)
has determined the amount of
carbon dioxide respired, by means
of a eudiometer, and has based
comparisons of sun and shade
plants upon the results. As he
points out, however, light has no
direct connection with respiration.
Although the latter increases
necessarily with increased
Fig. 39. Position of chloroplasts in nutrition, the relation between
aerial leaf (1) and submerged leaf (2) them is so obscure, and so far from
of Callitriche bifida. × 250. exact, that the amount of
respiration can in no wise serve as
a measure of the response to light.
As a result of the foregoing, it is clear that no functional response is able to
furnish a satisfactory measure of adjustment to light, though one or two
have perhaps sufficient value to warrant their use. Indeed, structural
adaptations offer a much better basis for the quantitative determination of
the effects of light stimuli, as will be shown later.
In attempting to use the number of chloroplasts or starch grains as a
measure of response, the study should be confined to the sun and shade
forms of the same species, or, in some cases, to the forms of closely related
Page 183
species. The margin of error is so great and the connection with light
sufficiently remote that comparisons between unrelated forms or species are
almost wholly without value. It has already been stated that starch is merely
the surplus carbohydrate not removed by translocation; the amount of
starch, even if accurately determined, can furnish no real clue to the amount
of glucose manufactured. In like manner, the number of chloroplasts can
furnish little more than an approximation of the amount of chlorophyll,
unless size and color are taken into account. In sun and shade ecads of the
same species, the general functional relations are essentially the same, and
whatever differences appear may properly be ascribed to different light
intensities for the two habitats. The actual counting of chloroplasts and
starch grains is a simple task. Pieces of the leaves of the two or more forms
to be compared are killed and imbedded in paraffin in the usual way. To
save time, the staining is done in toto. Methyl green is used for the
chloroplasts and a strong solution of iodine for the starch grains. When
counts are to be made of both, the leaves are first treated with iodin and
then stained with the methyl green. The thickness of the microtome sections
should be less than that of the palisade cells in order that the chloroplasts
may appear in profile, thus facilitating the counting. The count is made for a
segment 100 μ in width across the entire leaf. Two segments in different
parts of the section are counted, and the result multiplied by five to give the
number for a segment 1 millimeter in width. Although sun and shade leaves
regularly differ in size and thickness, no correction is necessary for these.
Size and thickness stand in reciprocal relation to each other in ecads, and
thickness is largely an expression of the absorption of light, and hence of its
intensity. In the gravel, forest, and thicket ecads of Galium boreale, counts
of the chloroplasts gave the following results. The gravel form (light 1)
showed 3,500 plastids in the 1–mm. segment, the forest form (light .03)
possessed 1,350, and the thicket form (light .002), 1,000. In these no
attention was paid to the size and form of the plastids in the different leaves,
since the differences were inappreciable. When this is not the case, both
factors should be taken into account. Starch grains are counted in exactly
the same way. Indeed, if care is taken to collect leaves of forms to be
compared, at approximately the same time on sunshiny days, a count of the
chloroplasts is equivalent to a count of the starch grains in the vast majority
of cases. Measurements of the size of starch grains can be made with
accuracy only when the leaves are killed in the field at the same time,
sufficiently remote that comparisons between unrelated forms or species are
almost wholly without value. It has already been stated that starch is merely
the surplus carbohydrate not removed by translocation; the amount of
starch, even if accurately determined, can furnish no real clue to the amount
of glucose manufactured. In like manner, the number of chloroplasts can
furnish little more than an approximation of the amount of chlorophyll,
unless size and color are taken into account. In sun and shade ecads of the
same species, the general functional relations are essentially the same, and
whatever differences appear may properly be ascribed to different light
intensities for the two habitats. The actual counting of chloroplasts and
starch grains is a simple task. Pieces of the leaves of the two or more forms
to be compared are killed and imbedded in paraffin in the usual way. To
save time, the staining is done in toto. Methyl green is used for the
chloroplasts and a strong solution of iodine for the starch grains. When
counts are to be made of both, the leaves are first treated with iodin and
then stained with the methyl green. The thickness of the microtome sections
should be less than that of the palisade cells in order that the chloroplasts
may appear in profile, thus facilitating the counting. The count is made for a
segment 100 μ in width across the entire leaf. Two segments in different
parts of the section are counted, and the result multiplied by five to give the
number for a segment 1 millimeter in width. Although sun and shade leaves
regularly differ in size and thickness, no correction is necessary for these.
Size and thickness stand in reciprocal relation to each other in ecads, and
thickness is largely an expression of the absorption of light, and hence of its
intensity. In the gravel, forest, and thicket ecads of Galium boreale, counts
of the chloroplasts gave the following results. The gravel form (light 1)
showed 3,500 plastids in the 1–mm. segment, the forest form (light .03)
possessed 1,350, and the thicket form (light .002), 1,000. In these no
attention was paid to the size and form of the plastids in the different leaves,
since the differences were inappreciable. When this is not the case, both
factors should be taken into account. Starch grains are counted in exactly
the same way. Indeed, if care is taken to collect leaves of forms to be
compared, at approximately the same time on sunshiny days, a count of the
chloroplasts is equivalent to a count of the starch grains in the vast majority
of cases. Measurements of the size of starch grains can be made with
accuracy only when the leaves are killed in the field at the same time,
Page 184
preferably in the afternoon. Counts of chloroplasts alone can be used as
measures of response in plants that produce sugar or oil, while either
chloroplasts or starch grains or both may be made the basis in starch-
forming leaves.
Hesselmann (l. c., 379) has employed Sachs’s iodine test as a measure of
photosynthesis. This has the advantage of permitting macroscopic
examination, but the comparison of the stained leaves can give only a very
general idea of the relative photosynthetic activity of two or more ecads.
The iodine test is made as follows:[17] fresh leaves are placed for a few
minutes in boiling water, and then in 95 per cent alcohol for 2–5 minutes, in
order to remove the chlorophyll and other soluble substances. The leaves
are placed in the iodine solution for ½–3 hours, or until no further change in
color takes place. The strength of the solution is not clearly indicated by
Sachs, who says: “I used an alcoholic solution of iodin which is best made
by dissolving a large quantity of iodin in strong alcohol and adding to this
sufficient distilled water to give the liquid the color of dark beer.” This
solution may be approximated by dissolving ⅓ gram of iodin in 100 grams
of 30 per cent alcohol. The stained leaves are put in a white porcelain dish
filled with distilled water, and the dish placed in the strong diffuse light of a
window. The colored leaf stands out sharply against the porcelain, and the
degree of coloration, and hence of starch content, is determined by the
following table:
1. bright yellow or leather yellow (no starch in the chlorenchym)
2. blackish (very little starch in the chlorenchym)
3. dull black (starch abundant)
4. coal black (starch very abundant)
5. black, with metallic luster (maximum starch-content)
ADAPTATION
177. Influence of chloroplasts upon form and structure. The
beginning of all modifications produced by light stimuli must be sought in
the chloroplast as the sensitized unit of the protoplasm. Hence, it seems a
measures of response in plants that produce sugar or oil, while either
chloroplasts or starch grains or both may be made the basis in starch-
forming leaves.
Hesselmann (l. c., 379) has employed Sachs’s iodine test as a measure of
photosynthesis. This has the advantage of permitting macroscopic
examination, but the comparison of the stained leaves can give only a very
general idea of the relative photosynthetic activity of two or more ecads.
The iodine test is made as follows:[17] fresh leaves are placed for a few
minutes in boiling water, and then in 95 per cent alcohol for 2–5 minutes, in
order to remove the chlorophyll and other soluble substances. The leaves
are placed in the iodine solution for ½–3 hours, or until no further change in
color takes place. The strength of the solution is not clearly indicated by
Sachs, who says: “I used an alcoholic solution of iodin which is best made
by dissolving a large quantity of iodin in strong alcohol and adding to this
sufficient distilled water to give the liquid the color of dark beer.” This
solution may be approximated by dissolving ⅓ gram of iodin in 100 grams
of 30 per cent alcohol. The stained leaves are put in a white porcelain dish
filled with distilled water, and the dish placed in the strong diffuse light of a
window. The colored leaf stands out sharply against the porcelain, and the
degree of coloration, and hence of starch content, is determined by the
following table:
1. bright yellow or leather yellow (no starch in the chlorenchym)
2. blackish (very little starch in the chlorenchym)
3. dull black (starch abundant)
4. coal black (starch very abundant)
5. black, with metallic luster (maximum starch-content)
ADAPTATION
177. Influence of chloroplasts upon form and structure. The
beginning of all modifications produced by light stimuli must be sought in
the chloroplast as the sensitized unit of the protoplasm. Hence, it seems a
Page 185
truism to say that the number and arrangement of the chloroplasts determine
the form of the cell, the tissue, and the leaf, although it has not yet been
possible to demonstrate this connection conclusively by means of
experiment. In spite of the lack of experimental proof, this principle is by
far the best guide through the subject of adaptations to light, and in the
discussion that follows, it is the fundamental hypothesis upon which all
others rest. The three propositions upon which this main hypothesis is
grounded are: (1) that the number of chloroplasts increases with the
intensity of the light; (2) that in shaded habitats chloroplasts arrange
themselves so as to increase the surface for receiving light; (3) that
chloroplasts in sunny habitats place themselves in such fashion as to
decrease the surface, and consequently the transpiration due to light. In
these, there can be little doubt concerning the facts of number and
arrangement, since they have been repeatedly verified. The purpose of
epistrophe and apostrophe, however, can not yet be stated with complete
certainty.
The stimulus of sunlight and of diffuse light is the same in one respect,
namely, the chloroplasts respond by arranging themselves in rows or lines
on the cell wall. The direct consequence of this is to polarize the cell, and
its form changes from globoid to oblong. This effect is felt more or less
equally by both palisade and sponge cells, but the disturbing influence of
aeration has caused the polarity of the cells to be much less conspicuous in
the sponge than in the palisade tissue. While the cells of both are typically
polarized, however, they assume very different positions with reference to
incident light. This position is directly dependent upon the arrangement of
the plastids as determined by the light intensity. In consequence, palisade
cells stand at right angles to the surface and parallel with the impinging
rays; the sponge cells, conversely, are parallel with the epidermis and at
right angles to the light ray. Some plants, especially monocotyledons,
exhibit little or no polarity in the chlorenchym. As a result the leaf does not
show a differentiation into sponge and palisade, and the leaves of sun and
shade ecads are essentially alike in form and structure. The form of the leaf
is largely determined by the chloroplasts acting through the cells that
contain them. A preponderance of sponge tissue produces an extension of
leaf in the direction determined by the arrangement of the plastids and the
shape of the sponge cells, viz., at right angles to the light. Shade leaves are
in consequence broader and thinner, and sometimes larger, than sun leaves
the form of the cell, the tissue, and the leaf, although it has not yet been
possible to demonstrate this connection conclusively by means of
experiment. In spite of the lack of experimental proof, this principle is by
far the best guide through the subject of adaptations to light, and in the
discussion that follows, it is the fundamental hypothesis upon which all
others rest. The three propositions upon which this main hypothesis is
grounded are: (1) that the number of chloroplasts increases with the
intensity of the light; (2) that in shaded habitats chloroplasts arrange
themselves so as to increase the surface for receiving light; (3) that
chloroplasts in sunny habitats place themselves in such fashion as to
decrease the surface, and consequently the transpiration due to light. In
these, there can be little doubt concerning the facts of number and
arrangement, since they have been repeatedly verified. The purpose of
epistrophe and apostrophe, however, can not yet be stated with complete
certainty.
The stimulus of sunlight and of diffuse light is the same in one respect,
namely, the chloroplasts respond by arranging themselves in rows or lines
on the cell wall. The direct consequence of this is to polarize the cell, and
its form changes from globoid to oblong. This effect is felt more or less
equally by both palisade and sponge cells, but the disturbing influence of
aeration has caused the polarity of the cells to be much less conspicuous in
the sponge than in the palisade tissue. While the cells of both are typically
polarized, however, they assume very different positions with reference to
incident light. This position is directly dependent upon the arrangement of
the plastids as determined by the light intensity. In consequence, palisade
cells stand at right angles to the surface and parallel with the impinging
rays; the sponge cells, conversely, are parallel with the epidermis and at
right angles to the light ray. Some plants, especially monocotyledons,
exhibit little or no polarity in the chlorenchym. As a result the leaf does not
show a differentiation into sponge and palisade, and the leaves of sun and
shade ecads are essentially alike in form and structure. The form of the leaf
is largely determined by the chloroplasts acting through the cells that
contain them. A preponderance of sponge tissue produces an extension of
leaf in the direction determined by the arrangement of the plastids and the
shape of the sponge cells, viz., at right angles to the light. Shade leaves are
in consequence broader and thinner, and sometimes larger, than sun leaves
Page 186
of the same species. A preponderance of palisade likewise results in the
extension of the leaf in the line of the plastids and the palisade cells, i. e., in
a direction parallel with the incident ray. In accordance, sun leaves are
thicker, narrower, and often smaller than shade leaves.
178. Form of leaves and stems. In outline, shade leaves are more nearly
entire than sun leaves. This statement is readily verified by the comparison
of sun and shade ecads, though the rule is by no means without exceptions.
In the leaf prints shown in figures 14 and 15, the modification of form is
well shown in Bursa and Thalictrum; in Capnoides the change is less
evident, while in Achilleia and Machaeranthera lobing is more pronounced
in the shade form, a fact which is, however, readily explained when other
factors are taken into account. The leaf prints cited serve as more
satisfactory examples of the increase of size in consequence of an increase
in the surface of the shade leaf, although the leaves printed were selected
solely with reference to thickness and size or outline. In all comparisons of
this kind, however, the relative size and vigor of the two plants must be
taken into account. This precaution is likewise necessary in the case of
thickness, which should always be considered in connection with amount of
surface. The relation between surface and thickness is shown by the
following species, in all of which the size of the leaf is greater in the shade
than in the sun. In Capnoides aureum, the thickness of the shade leaf is ½ (6
: 12) that of the sun leaf; in Galium boreale the ratio is 5 : 12, and in
Allionia linearis it is 3 : 12. The ratio in Thalictrum sparsiflorum is 9 : 12,
and in Machaeranthera aspera 11 : 12. The thickness of sun and shade
leaves of Bursa bursa-pastoris is as 14 : 12, but this anomaly is readily
explained by the size of the plants; the shade form is ten times larger than
the sun form. Certain species, e. g., Erigeron speciosus, Potentilla
bipinnatifida, etc., show no change in thickness and but little modification
in size or outline. They furnish additional evidence of a fundamental
principle in adaptation, namely that the amount of structural response is
profoundly affected by the stability of the ancestral type.
The effect of diffuse light in causing stems to elongate, though known for
a long time, is still unexplained. The old explanation that the plant stretches
up to obtain more light seems to be based upon nothing more than the
coincidence that the light comes from the direction toward which the stem
grows. Later researches have shown that the stretching of the stem is due to
the excessive elongation of the parenchyma cells, but the cause of the latter
extension of the leaf in the line of the plastids and the palisade cells, i. e., in
a direction parallel with the incident ray. In accordance, sun leaves are
thicker, narrower, and often smaller than shade leaves.
178. Form of leaves and stems. In outline, shade leaves are more nearly
entire than sun leaves. This statement is readily verified by the comparison
of sun and shade ecads, though the rule is by no means without exceptions.
In the leaf prints shown in figures 14 and 15, the modification of form is
well shown in Bursa and Thalictrum; in Capnoides the change is less
evident, while in Achilleia and Machaeranthera lobing is more pronounced
in the shade form, a fact which is, however, readily explained when other
factors are taken into account. The leaf prints cited serve as more
satisfactory examples of the increase of size in consequence of an increase
in the surface of the shade leaf, although the leaves printed were selected
solely with reference to thickness and size or outline. In all comparisons of
this kind, however, the relative size and vigor of the two plants must be
taken into account. This precaution is likewise necessary in the case of
thickness, which should always be considered in connection with amount of
surface. The relation between surface and thickness is shown by the
following species, in all of which the size of the leaf is greater in the shade
than in the sun. In Capnoides aureum, the thickness of the shade leaf is ½ (6
: 12) that of the sun leaf; in Galium boreale the ratio is 5 : 12, and in
Allionia linearis it is 3 : 12. The ratio in Thalictrum sparsiflorum is 9 : 12,
and in Machaeranthera aspera 11 : 12. The thickness of sun and shade
leaves of Bursa bursa-pastoris is as 14 : 12, but this anomaly is readily
explained by the size of the plants; the shade form is ten times larger than
the sun form. Certain species, e. g., Erigeron speciosus, Potentilla
bipinnatifida, etc., show no change in thickness and but little modification
in size or outline. They furnish additional evidence of a fundamental
principle in adaptation, namely that the amount of structural response is
profoundly affected by the stability of the ancestral type.
The effect of diffuse light in causing stems to elongate, though known for
a long time, is still unexplained. The old explanation that the plant stretches
up to obtain more light seems to be based upon nothing more than the
coincidence that the light comes from the direction toward which the stem
grows. Later researches have shown that the stretching of the stem is due to
the excessive elongation of the parenchyma cells, but the cause of the latter
Page 187
is far from apparent. It is generally assumed to be due to a lack of the tonic
action of sunlight, which brings about a retardation of growth in sun plants.
The evidence in favor of this view is not conclusive, and it seems probable
at least that the elongation of the parenchyma cells takes place under
conditions which favor the mechanical stretching of the cell wall, but
inhibit the proper growth of the wall by intussusception. It is hardly
necessary to state that the reduced photosynthetic activity of shade plants
favors such an explanation. Whatever the cause, the advantage that results
from the elongation of the internodes is apparent. Leaves interfere less with
the illumination of those below them, and the leaves of the branches are
carried away from the stem in such a way as to give the plant the best
possible exposure for its aggregate leaf surface.
179. Modification of the epidermis. The development of epidermal
chloroplasts in diffuse light is the only change which is due to the direct
effect of light. This does not often occur in the shade ecads of sun species,
but chloroplasts are regularly present in the epidermis of woodland ferns
and of submerged plants. The slight development of hairs in sciophilous
plants is an advantage, but it must be referred to the factors that determine
water loss. The significance of epidermal papillae in increasing the
absorption of light by shade plants has already been discussed. The
questions as to what factor has called forth these papillae and what purpose
they serve must still be regarded as unsettled. The increased size of the
epidermal cells, which is a fairly constant feature of shade ecads, seems to
be for the purpose of increasing translocation and transpiration, and to bear
no relation to light. The extreme development of the cells of the epidermis
in Streptopus and Limnorchis, which grow at the edge of mountain brooks,
has been plausibly explained by E. S. Clements as a contrivance to increase
water loss. The presence of a waxy coating, such as that found upon the
leaves of Impatiens aurea and I. pallida, is clearly to prevent the wetting of
the leaf and the consequent stoppage of the stomata. In regard to the latter,
different observers have noted that the number of the stomata is greater in
sun than in shade leaves. This holds generally for sun and shade species, but
it is most clearly indicated by different ecads of the same species. In
Scutellaria brittonii, the sun form possesses 100 stomata per square
millimeter, but in the shade these are reduced to 40 per square millimeter;
the sun leaf of Allionia linearis has 180 stomata to the square millimeter,
the shade leaf 90. In the stable leaf of Erigeron speciosus, however, the
action of sunlight, which brings about a retardation of growth in sun plants.
The evidence in favor of this view is not conclusive, and it seems probable
at least that the elongation of the parenchyma cells takes place under
conditions which favor the mechanical stretching of the cell wall, but
inhibit the proper growth of the wall by intussusception. It is hardly
necessary to state that the reduced photosynthetic activity of shade plants
favors such an explanation. Whatever the cause, the advantage that results
from the elongation of the internodes is apparent. Leaves interfere less with
the illumination of those below them, and the leaves of the branches are
carried away from the stem in such a way as to give the plant the best
possible exposure for its aggregate leaf surface.
179. Modification of the epidermis. The development of epidermal
chloroplasts in diffuse light is the only change which is due to the direct
effect of light. This does not often occur in the shade ecads of sun species,
but chloroplasts are regularly present in the epidermis of woodland ferns
and of submerged plants. The slight development of hairs in sciophilous
plants is an advantage, but it must be referred to the factors that determine
water loss. The significance of epidermal papillae in increasing the
absorption of light by shade plants has already been discussed. The
questions as to what factor has called forth these papillae and what purpose
they serve must still be regarded as unsettled. The increased size of the
epidermal cells, which is a fairly constant feature of shade ecads, seems to
be for the purpose of increasing translocation and transpiration, and to bear
no relation to light. The extreme development of the cells of the epidermis
in Streptopus and Limnorchis, which grow at the edge of mountain brooks,
has been plausibly explained by E. S. Clements as a contrivance to increase
water loss. The presence of a waxy coating, such as that found upon the
leaves of Impatiens aurea and I. pallida, is clearly to prevent the wetting of
the leaf and the consequent stoppage of the stomata. In regard to the latter,
different observers have noted that the number of the stomata is greater in
sun than in shade leaves. This holds generally for sun and shade species, but
it is most clearly indicated by different ecads of the same species. In
Scutellaria brittonii, the sun form possesses 100 stomata per square
millimeter, but in the shade these are reduced to 40 per square millimeter;
the sun leaf of Allionia linearis has 180 stomata to the square millimeter,
the shade leaf 90. In the stable leaf of Erigeron speciosus, however, the
Page 188
number of stomata is the same, 180
per square millimeter, for sunlight
and for diffuse light. The presence
of the larger number of stomata in
the plant exposed to greater loss,
which at first thought seems
startling, is readily explained by
the more intense photosynthetic
activity in the sun. Since the
absorption of gases is the primary
function of the stomata, and
transpiration merely secondary, it
is evident that sun plants must have
more stomata than shade plants.
This is further explained by the
fact that the small air passages of
sun leaves necessitate frequent
inlets, which are less necessary in
shade leaves with their larger air
spaces. In shade plants, moreover,
the decrease in the number is
compensated in some measure by
the ability of the epidermal cells to
absorb gases directly from the air.
180. The differentiation of the
chlorenchym. The division of the
chlorenchym into two tissues,
sponge and palisade, is the normal
consequence of the unequal Fig. 40. Isophotophyll of Allionia
illumination of the leaf surfaces. linearis, showing diphotic ecads: 1,
Exceptions to this rule occur only light 1; 2, light .012; 3, light .003. ×
in certain monocotyledons, in 130.
which the leaf tissue consists of
sponge-like cells throughout, and
in those stable species that retain more or less palisade in spite of their
change to diffuse light. The difference in the illumination of the two
surfaces is determined by the position of the leaf. Leaves that are erect or
per square millimeter, for sunlight
and for diffuse light. The presence
of the larger number of stomata in
the plant exposed to greater loss,
which at first thought seems
startling, is readily explained by
the more intense photosynthetic
activity in the sun. Since the
absorption of gases is the primary
function of the stomata, and
transpiration merely secondary, it
is evident that sun plants must have
more stomata than shade plants.
This is further explained by the
fact that the small air passages of
sun leaves necessitate frequent
inlets, which are less necessary in
shade leaves with their larger air
spaces. In shade plants, moreover,
the decrease in the number is
compensated in some measure by
the ability of the epidermal cells to
absorb gases directly from the air.
180. The differentiation of the
chlorenchym. The division of the
chlorenchym into two tissues,
sponge and palisade, is the normal
consequence of the unequal Fig. 40. Isophotophyll of Allionia
illumination of the leaf surfaces. linearis, showing diphotic ecads: 1,
Exceptions to this rule occur only light 1; 2, light .012; 3, light .003. ×
in certain monocotyledons, in 130.
which the leaf tissue consists of
sponge-like cells throughout, and
in those stable species that retain more or less palisade in spite of their
change to diffuse light. The difference in the illumination of the two
surfaces is determined by the position of the leaf. Leaves that are erect or
Page 189
nearly so usually have both sides
about equally illuminated, and they
may be termed isophotic. Leaves
that stand more or less at right
angles to the stem receive much
more light upon the upper surface
than upon the lower, and may
accordingly be termed diphotic.
Certain dorsiventral leaves,
however, absorb practically as
much light on the lower side as
upon the upper. This is true of sun
leaves with a dense hairy covering,
which screens out the greater part
of the light incident upon the upper
surface. It occurs also in
xerophytes which grow in light-
colored sands and gravels that
Fig. 41. Isophotophyll of Helianthus serve to reflect the sun’s rays upon
pumilus, showing isophotic ecad: 1, the lower surface. In deep shade,
sun leaf; 2, shade leaf (light .012). × moreover, there is no essential
130. difference in the intensity of the
light received by the two surfaces,
and shade leaves are often
isophotic in consequence. From
these examples it is evident that
isophotic and diphotic leaves occur
in both sun and shade, and that the
intensity of the light is secondary
to direction, in so far as the
modification of the leaf is
concerned.
The essential connection of
sponge tissue with diffuse light is
conclusively shown by the
Fig. 42. Diphotophylls of Quercus behavior of shade ecads, but
novimexicana: 1, sun leaf; 2, shade further evidence of great value is
about equally illuminated, and they
may be termed isophotic. Leaves
that stand more or less at right
angles to the stem receive much
more light upon the upper surface
than upon the lower, and may
accordingly be termed diphotic.
Certain dorsiventral leaves,
however, absorb practically as
much light on the lower side as
upon the upper. This is true of sun
leaves with a dense hairy covering,
which screens out the greater part
of the light incident upon the upper
surface. It occurs also in
xerophytes which grow in light-
colored sands and gravels that
Fig. 41. Isophotophyll of Helianthus serve to reflect the sun’s rays upon
pumilus, showing isophotic ecad: 1, the lower surface. In deep shade,
sun leaf; 2, shade leaf (light .012). × moreover, there is no essential
130. difference in the intensity of the
light received by the two surfaces,
and shade leaves are often
isophotic in consequence. From
these examples it is evident that
isophotic and diphotic leaves occur
in both sun and shade, and that the
intensity of the light is secondary
to direction, in so far as the
modification of the leaf is
concerned.
The essential connection of
sponge tissue with diffuse light is
conclusively shown by the
Fig. 42. Diphotophylls of Quercus behavior of shade ecads, but
novimexicana: 1, sun leaf; 2, shade further evidence of great value is
Page 190
leaf of the same tree (light .06). × furnished by diphotic leaves, and
130. those with hairy coverings. The
sponge tissue, which in the shade
leaf is due to the diffuse light of the habitat, is produced in the hairy leaf as
a consequence of the absorption and diffraction of the light by the covering.
In ordinary diphotic leaves, the absorption of light in the palisade reduces
the intensity to such a degree that the cells of the lower half of the leaf are
in diffuse light, and are in consequence modified to form sponge tissue. The
sponge tissue of the diphotic leaf is just as clearly an adaptation to diffuse
light as it is in those plants where the whole chlorenchym is in the shade of
other plants or of a covering of hairs. As is indicated later, all these relations
permit of ready confirmation by experiment, either by changing the position
of the leaf or by modifying the intensity or direction of the light.
The preceding discussion makes
it fairly clear that sponge tissue is
developed primarily to increase the
light-absorbing surface. Because of
its direct connection with
photosynthesis, the sponge tissue is
the especial organ of aeration, also,
and since it shows a high
development of air spaces for this
purpose, it is inevitably concerned
in transpiration. It seems to be
partly a coincidence, however, that
the sponge is found next to the
lower surface upon which the
stomata are most numerous. This is
indicated by artificial ecads of
Ranunculus sceleratus, in which
sponge tissue is unusually
developed, although the stomata
Fig. 43. A plastic species, Mertensia
are much more numerous upon the
polyphylla, showing the effect of
upper surface. Palisade tissue is
water upon the sponge: 1, chresard
apparently developed primarily as
25%; 2, chresard 12%. × 130.
a protection against water loss,
particularly that due to the
130. those with hairy coverings. The
sponge tissue, which in the shade
leaf is due to the diffuse light of the habitat, is produced in the hairy leaf as
a consequence of the absorption and diffraction of the light by the covering.
In ordinary diphotic leaves, the absorption of light in the palisade reduces
the intensity to such a degree that the cells of the lower half of the leaf are
in diffuse light, and are in consequence modified to form sponge tissue. The
sponge tissue of the diphotic leaf is just as clearly an adaptation to diffuse
light as it is in those plants where the whole chlorenchym is in the shade of
other plants or of a covering of hairs. As is indicated later, all these relations
permit of ready confirmation by experiment, either by changing the position
of the leaf or by modifying the intensity or direction of the light.
The preceding discussion makes
it fairly clear that sponge tissue is
developed primarily to increase the
light-absorbing surface. Because of
its direct connection with
photosynthesis, the sponge tissue is
the especial organ of aeration, also,
and since it shows a high
development of air spaces for this
purpose, it is inevitably concerned
in transpiration. It seems to be
partly a coincidence, however, that
the sponge is found next to the
lower surface upon which the
stomata are most numerous. This is
indicated by artificial ecads of
Ranunculus sceleratus, in which
sponge tissue is unusually
developed, although the stomata
Fig. 43. A plastic species, Mertensia
are much more numerous upon the
polyphylla, showing the effect of
upper surface. Palisade tissue is
water upon the sponge: 1, chresard
apparently developed primarily as
25%; 2, chresard 12%. × 130.
a protection against water loss,
particularly that due to the
Page 191
absorption of light by the chloroplast. The small size of the intercellular
passages between palisade cells likewise aids in decreasing transpiration.
The fact that leaves with much palisade tissue transpire twice as much as
shade leaves is hardly an objection to this view, as Hesselmann (l. c., 442)
would think. It is readily explained by the intense photosynthesis of sun
plants, which makes necessary an increase, usually a doubling, in the
number of stomata, in consequence of which the transpiration is increased.
181. Types of leaves. Isophotic
leaves are equally illuminated and
possess more or less uniform
chlorenchym. Diphotic leaves are
unequally illuminated, and exhibit
a differentiation into palisade and
sponge tissues. They may be
distinguished as isophotophylls and
diphotophylls respectively.[18]
Isophotic leaves fall into three
types based upon the intensity of
the light. The staurophyll, or
palisade leaf, is a sun type in which
the equal illumination is due to the
upright position or to the reflection
from a light soil, and in which the
chlorenchym consists wholly of
rows of palisade cells. The
diplophyll is a special form of this
type in which the intense light does
not penetrate to the middle of the
Fig. 44. A stable species, Erigeron leaf, thus resulting in a central
speciosus: 1, sun leaf; 2, shade leaf sponge tissue, or water-storage
(light .03). × 130. tissue. The spongophyll, or sponge
leaf, is regularly a shade type; the
chlorenchym consists of sponge
cells alone. For the present at least it is also necessary to refer to this group
those monocotyledons which grow in the sun but contain no palisade tissue.
Diphotic leaves always contain both palisade and sponge, though the ratio
between them varies considerably. Diphotophylls are characteristic of sunny
passages between palisade cells likewise aids in decreasing transpiration.
The fact that leaves with much palisade tissue transpire twice as much as
shade leaves is hardly an objection to this view, as Hesselmann (l. c., 442)
would think. It is readily explained by the intense photosynthesis of sun
plants, which makes necessary an increase, usually a doubling, in the
number of stomata, in consequence of which the transpiration is increased.
181. Types of leaves. Isophotic
leaves are equally illuminated and
possess more or less uniform
chlorenchym. Diphotic leaves are
unequally illuminated, and exhibit
a differentiation into palisade and
sponge tissues. They may be
distinguished as isophotophylls and
diphotophylls respectively.[18]
Isophotic leaves fall into three
types based upon the intensity of
the light. The staurophyll, or
palisade leaf, is a sun type in which
the equal illumination is due to the
upright position or to the reflection
from a light soil, and in which the
chlorenchym consists wholly of
rows of palisade cells. The
diplophyll is a special form of this
type in which the intense light does
not penetrate to the middle of the
Fig. 44. A stable species, Erigeron leaf, thus resulting in a central
speciosus: 1, sun leaf; 2, shade leaf sponge tissue, or water-storage
(light .03). × 130. tissue. The spongophyll, or sponge
leaf, is regularly a shade type; the
chlorenchym consists of sponge
cells alone. For the present at least it is also necessary to refer to this group
those monocotyledons which grow in the sun but contain no palisade tissue.
Diphotic leaves always contain both palisade and sponge, though the ratio
between them varies considerably. Diphotophylls are characteristic of sunny
Page 192
mesophytic habitats. They are
frequent in xerophytic habitats as
well as in woodlands where the
light is not too diffuse. In the case
of stable species, this type of
structure sometimes persists in the
diffuse light of coniferous forests.
Floating leaves, in which the light
is almost completely cut off from
the lower surface, are also
members of this group. Submerged Fig. 45. Spongophyll of Gyrostachys
leaves, on the other hand, are stricta (light 1). × 130.
spongophylls.
182. Heliophytes and sciophytes. The great majority of sun plants
possess diphotophylls. This type is represented by Pedicularis procera (fig.
32). Plants with isophotophylls are found chiefly in xerophytic places,
though erect leaves of this type occur in most sunny habitats. The
staurophyll, in which the protection is due to the extreme development of
palisade tissue, is illustrated by Allionia linearis (fig. 40) and Bahia
dissecta (fig. 33). The diplophyll, which is characterized by a central band
of sponge tissue or storage cells, is found in Mertensia linearis (fig. 34).
The form of the spongophyll that is found in certain monocotyledons is
shown by Gyrostachys stricta (fig. 45). The spongophyll (fig. 38:3, 39:2) is
frequent among plants of deep shade, but as the leaf sections of Allionia
(figs. 38, 40) and Quercus (fig. 42) show, the diphotophyll is the rule in
shade ecads.
Experimental Evolution
183. Scope. The primary task of experimental evolution is the detailed
study, under measured conditions, of the origin of new forms in nature. As a
department of botanical research that is as yet unformed, it has little
concern with the host of hypotheses and theories which rest merely upon
general observation and conjecture. A few of these constitute good working
hypotheses or serve to indicate possible points of attack, but the vast
majority are worthless impedimenta which should be thrown away at the
frequent in xerophytic habitats as
well as in woodlands where the
light is not too diffuse. In the case
of stable species, this type of
structure sometimes persists in the
diffuse light of coniferous forests.
Floating leaves, in which the light
is almost completely cut off from
the lower surface, are also
members of this group. Submerged Fig. 45. Spongophyll of Gyrostachys
leaves, on the other hand, are stricta (light 1). × 130.
spongophylls.
182. Heliophytes and sciophytes. The great majority of sun plants
possess diphotophylls. This type is represented by Pedicularis procera (fig.
32). Plants with isophotophylls are found chiefly in xerophytic places,
though erect leaves of this type occur in most sunny habitats. The
staurophyll, in which the protection is due to the extreme development of
palisade tissue, is illustrated by Allionia linearis (fig. 40) and Bahia
dissecta (fig. 33). The diplophyll, which is characterized by a central band
of sponge tissue or storage cells, is found in Mertensia linearis (fig. 34).
The form of the spongophyll that is found in certain monocotyledons is
shown by Gyrostachys stricta (fig. 45). The spongophyll (fig. 38:3, 39:2) is
frequent among plants of deep shade, but as the leaf sections of Allionia
(figs. 38, 40) and Quercus (fig. 42) show, the diphotophyll is the rule in
shade ecads.
Experimental Evolution
183. Scope. The primary task of experimental evolution is the detailed
study, under measured conditions, of the origin of new forms in nature. As a
department of botanical research that is as yet unformed, it has little
concern with the host of hypotheses and theories which rest merely upon
general observation and conjecture. A few of these constitute good working
hypotheses or serve to indicate possible points of attack, but the vast
majority are worthless impedimenta which should be thrown away at the
Page 193
start. It is the general practice to speak of evolution as founded upon a solid
basis of incontestible facts, but a cursory examination of the evidence
shows that it is drawn, almost without exception, from observation alone,
and has in consequence suffered severely from interpretation. With the
exception of De Vries’s work on mutation, sustained and accurate
investigation of the evolution of plants has been lacking. As a result,
botanical research has been built high upon an insecure foundation, nearly
every stone of which must be carefully tested before it can be left
permanently in place. In a field so vast and important as evolution,
experiment should far outrun induction, and deduction should enter only
when it can show the way to a working hypothesis of real merit. The great
value of De Vries’s study of mutation as an example of the proper
experimental study of evolution has been seriously reduced by the fact that
the “mutation theory” has carried induction far beyond the warrant afforded
by experiment. The investigator who plans to make a serious study by
experiment of the origin of new plant forms should rest secure in the
conviction that the most rapid and certain progress can be made only by the
accumulation of a large number of unimpeachable facts, obtained by the
most exact methods of experimental study.
The general application of field experiment to evolution will render the
current methods of recognizing species quite useless. It will become
imperative to establish an experimental test for forms and species, and to
apply this test critically to every “new species.” Descriptive botany, as
practiced at present, will fall into disuse, as scientific standards come to
prevail, and in its place will appear a real science of taxonomy. In the latter
the criteria upon which species are based will be obtained solely by
experiment.
184. Fundamental lines of inquiry. There are two primary and sharply
defined fields of research in experimental evolution, namely, adaptation in
consequence of variation (and mutation), and hybridization. The latter
constitutes a particular field of inquiry, which is not intimately connected
with the problems of evolution in nature. In the study of specific adaptation,
two questions of profound importance appear. One deals with the effects of
ancestral fixity or plasticity in determining the amount of modification
produced by the habitat. These are fundamental problems, and a solution of
them can not be hoped for until exact and trustworthy data have been
provided by numerous experimental researches. It thus becomes clear that
basis of incontestible facts, but a cursory examination of the evidence
shows that it is drawn, almost without exception, from observation alone,
and has in consequence suffered severely from interpretation. With the
exception of De Vries’s work on mutation, sustained and accurate
investigation of the evolution of plants has been lacking. As a result,
botanical research has been built high upon an insecure foundation, nearly
every stone of which must be carefully tested before it can be left
permanently in place. In a field so vast and important as evolution,
experiment should far outrun induction, and deduction should enter only
when it can show the way to a working hypothesis of real merit. The great
value of De Vries’s study of mutation as an example of the proper
experimental study of evolution has been seriously reduced by the fact that
the “mutation theory” has carried induction far beyond the warrant afforded
by experiment. The investigator who plans to make a serious study by
experiment of the origin of new plant forms should rest secure in the
conviction that the most rapid and certain progress can be made only by the
accumulation of a large number of unimpeachable facts, obtained by the
most exact methods of experimental study.
The general application of field experiment to evolution will render the
current methods of recognizing species quite useless. It will become
imperative to establish an experimental test for forms and species, and to
apply this test critically to every “new species.” Descriptive botany, as
practiced at present, will fall into disuse, as scientific standards come to
prevail, and in its place will appear a real science of taxonomy. In the latter
the criteria upon which species are based will be obtained solely by
experiment.
184. Fundamental lines of inquiry. There are two primary and sharply
defined fields of research in experimental evolution, namely, adaptation in
consequence of variation (and mutation), and hybridization. The latter
constitutes a particular field of inquiry, which is not intimately connected
with the problems of evolution in nature. In the study of specific adaptation,
two questions of profound importance appear. One deals with the effects of
ancestral fixity or plasticity in determining the amount of modification
produced by the habitat. These are fundamental problems, and a solution of
them can not be hoped for until exact and trustworthy data have been
provided by numerous experimental researches. It thus becomes clear that
Page 194
the principal, if not the sole task of experimental evolution for years to
come is the diligent prosecution of accurate and prolonged experiment in
the modification of plant forms. It seems inevitable that this will be carried
on along the lines that have already been indicated. Plants will be grown in
habitats of measured value, or in different intensities of the same factor. The
relation between stimulus and adjustment will form the basis of careful
quantitative study, and the final expression of this relation in structural
modifications will find an exact record in drawings, photographs, exsiccati,
and biometrical measures. The making of an accurate and complete record
of the whole course of each experiment of this sort is an obligation that
rests upon every investigator. Studies in experimental evolution will prove
time-consuming beyond all other lines of botanical research, and the work
of one generation should appear in a record so perfect that it can be used
without doubt or hesitation as a basis for the studies of the succeeding
generation.
185. Ancestral form and structure. The significance of the fact that
some species have been found to remain unaltered structurally under
changes of habitats that produced striking modifications in others has
already been commented upon. It is hardly necessary to indicate the
important bearing which this has upon evolution. The very ability of a plant
to undergo modification, and hence to give rise to new forms, depends upon
the degree of fixity of the characters which it has inherited. Stable plants are
less susceptible of evolution than plastic ones. The latter adapt themselves
to new habitats with ease, and in each produce a new form, which may
serve as the starting point of a phylum. There is at present no clue whatever
as to what calls forth this essential difference in behavior. This is not
surprising in view of the fact that there have been no comparative
experimental studies of stable and plastic species. Until these have been
made, it is impossible to do more than to formulate a working hypothesis as
to the effect of stability, and an explanation of the forces which cause or
control it is altogether out of the question.
186. Variation and mutation. New forms of plants are known to arise
by three methods, viz., variation, mutation, adaptation. The evidence in
support of these is almost wholly observational, and consequently more or
less inexact, but for each there exist a few accurate experiments which are
conclusive. Origin by variation and subsequent selection is the essence of
the Darwinian theory of the origin of species. According to this the
come is the diligent prosecution of accurate and prolonged experiment in
the modification of plant forms. It seems inevitable that this will be carried
on along the lines that have already been indicated. Plants will be grown in
habitats of measured value, or in different intensities of the same factor. The
relation between stimulus and adjustment will form the basis of careful
quantitative study, and the final expression of this relation in structural
modifications will find an exact record in drawings, photographs, exsiccati,
and biometrical measures. The making of an accurate and complete record
of the whole course of each experiment of this sort is an obligation that
rests upon every investigator. Studies in experimental evolution will prove
time-consuming beyond all other lines of botanical research, and the work
of one generation should appear in a record so perfect that it can be used
without doubt or hesitation as a basis for the studies of the succeeding
generation.
185. Ancestral form and structure. The significance of the fact that
some species have been found to remain unaltered structurally under
changes of habitats that produced striking modifications in others has
already been commented upon. It is hardly necessary to indicate the
important bearing which this has upon evolution. The very ability of a plant
to undergo modification, and hence to give rise to new forms, depends upon
the degree of fixity of the characters which it has inherited. Stable plants are
less susceptible of evolution than plastic ones. The latter adapt themselves
to new habitats with ease, and in each produce a new form, which may
serve as the starting point of a phylum. There is at present no clue whatever
as to what calls forth this essential difference in behavior. This is not
surprising in view of the fact that there have been no comparative
experimental studies of stable and plastic species. Until these have been
made, it is impossible to do more than to formulate a working hypothesis as
to the effect of stability, and an explanation of the forces which cause or
control it is altogether out of the question.
186. Variation and mutation. New forms of plants are known to arise
by three methods, viz., variation, mutation, adaptation. The evidence in
support of these is almost wholly observational, and consequently more or
less inexact, but for each there exist a few accurate experiments which are
conclusive. Origin by variation and subsequent selection is the essence of
the Darwinian theory of the origin of species. According to this the
Page 195
appearance of a new form is due to the accumulation, and selection, through
a long period, of minute differences which prove advantageous to the plant
in its competition with others in nature, or are desirable under cultivation.
Slight variations appear indiscriminately in every species. Their cause is not
known, but since they are found even in the most uniform habitats, it is
impossible to find any direct connection between them and the physical
factors. In the case of origin by mutation, the new form appears suddenly,
with definite characteristics fully developed. Selection, in the usual sense of
the term, does not enter into mutation at all, though the persistence of the
new form is still to be determined by competition. Mutations are known at
present for only a few species, and their actual appearance has been studied
in a very few cases. Like variations, they are indiscriminate in character.
The chief difference between them is apparently one of degree. Indeed,
mutation lends itself readily to the hypothesis that it is simply the sudden
appearance of latent variations which have accumulated within the plant.
De Vries regards constancy as an essential feature of mutation, but the
evidence from the mutants of Onagra is not convincing. Indeed, while there
can be no question of the occurrence of mutation in plants, a fact known for
many years, the facts so far brought forward in support of the “mutation
theory” fall far short of proving “the lack of significance of individual
variability, and the high value of mutability for the origin of species.”[19]
Mutations do not show any direct connection with the habitat, but their
sudden appearance suggests that they may be latent or delayed responses to
the ordinary stimuli. Origin by adaptation is the immediate consequence of
the stimuli exerted by the physical factors of a habitat. This fact
distinguishes it from origin by variation, or by mutation. The new form may
appear suddenly, often in a single generation, or gradually, but in either case
it is the result of adaptation that is necessarily advantageous, because it is
the result of adjustment to controlling physical factors. Origin by adaptation
is perhaps only a special kind of origin by variation, but this might be said
with equal truth of mutation. New forms resulting from adaptation are like
those produced from mutation, in that they appear suddenly as a rule and
without the agency of selection. They are essentially different, inasmuch as
their cause may be found at once in the habitat, and since a reversal of
stimuli produces, in many cases at least, a reversion in form and structure to
the ancestral type.
a long period, of minute differences which prove advantageous to the plant
in its competition with others in nature, or are desirable under cultivation.
Slight variations appear indiscriminately in every species. Their cause is not
known, but since they are found even in the most uniform habitats, it is
impossible to find any direct connection between them and the physical
factors. In the case of origin by mutation, the new form appears suddenly,
with definite characteristics fully developed. Selection, in the usual sense of
the term, does not enter into mutation at all, though the persistence of the
new form is still to be determined by competition. Mutations are known at
present for only a few species, and their actual appearance has been studied
in a very few cases. Like variations, they are indiscriminate in character.
The chief difference between them is apparently one of degree. Indeed,
mutation lends itself readily to the hypothesis that it is simply the sudden
appearance of latent variations which have accumulated within the plant.
De Vries regards constancy as an essential feature of mutation, but the
evidence from the mutants of Onagra is not convincing. Indeed, while there
can be no question of the occurrence of mutation in plants, a fact known for
many years, the facts so far brought forward in support of the “mutation
theory” fall far short of proving “the lack of significance of individual
variability, and the high value of mutability for the origin of species.”[19]
Mutations do not show any direct connection with the habitat, but their
sudden appearance suggests that they may be latent or delayed responses to
the ordinary stimuli. Origin by adaptation is the immediate consequence of
the stimuli exerted by the physical factors of a habitat. This fact
distinguishes it from origin by variation, or by mutation. The new form may
appear suddenly, often in a single generation, or gradually, but in either case
it is the result of adaptation that is necessarily advantageous, because it is
the result of adjustment to controlling physical factors. Origin by adaptation
is perhaps only a special kind of origin by variation, but this might be said
with equal truth of mutation. New forms resulting from adaptation are like
those produced from mutation, in that they appear suddenly as a rule and
without the agency of selection. They are essentially different, inasmuch as
their cause may be found at once in the habitat, and since a reversal of
stimuli produces, in many cases at least, a reversion in form and structure to
the ancestral type.
Page 196
A valid distinction between forms or species upon the basis of constancy
is impracticable at the present time. It is doubtful that such a distinction can
ever be made in anything like an absolute sense, since all degrees of
fluctuation may be observed between constancy and inconstancy. In all
events, it is gratuitous to make constancy the essential criterion in the
present state of our knowledge. So little is certainly known of it that it is
equally unscientific to affirm or to deny its value, and even a tentative
statement can not be ventured until a vast amount of evidence has been
obtained from experiment. Accordingly, there is absolutely no warrant,
other than tradition, for limiting the term species to a constant group. In the
evolutionary sense, a species is the aggregate ancestral group and the new
forms which have sprung from it by variation, mutation, or adaptation. It
should not be regarded as an isolated unit for purposes of descriptive
botany; indeed, its use in this connection is purely secondary. It is properly
the unit to be used in indicating the primary relationships which are the
result of evolution.
On the basis of their actual behavior in the production of new forms,
species may be distinguished as variable, mutable, or adaptable. The new
form which results from variation is a variant; the product of mutation is a
mutant, and that of adaptation, an ecad. The following examples serve to
illustrate these distinctions. Machaeranthera canescens, judging from the
numerous minute intergrades between its many forms, is a variable species,
i. e., one in which forms are arising by the gradual selection of small
variations. It apparently comprises a large number of variants, M. canescens
aspera, superba, ramosa, viscosa, etc. Onagra lamarckiana is a mutable
species: it comprises many mutants, e. g., Onagra lamarckiana gigas, O. l.
nanella, O. l. lata, etc. Galium boreale is an adaptable species: it possesses
one distinct ecad, Galium boreale hylocolum, which is the shade form of the
species.
187. Methods. The best of all experiments in evolution are those that are
constantly being made in nature. Such experiments are readily discovered
and studied in the case of origin by adaptation; variants present much
greater difficulties, while mutants are very rare under natural conditions.
The method which makes use of these experiments may be termed the
method of natural experiment. The number of ecads which appear naturally
in vegetation is limited, however, and it is consequently very desirable to
produce them artificially, by the method of habitat culture. This method,
is impracticable at the present time. It is doubtful that such a distinction can
ever be made in anything like an absolute sense, since all degrees of
fluctuation may be observed between constancy and inconstancy. In all
events, it is gratuitous to make constancy the essential criterion in the
present state of our knowledge. So little is certainly known of it that it is
equally unscientific to affirm or to deny its value, and even a tentative
statement can not be ventured until a vast amount of evidence has been
obtained from experiment. Accordingly, there is absolutely no warrant,
other than tradition, for limiting the term species to a constant group. In the
evolutionary sense, a species is the aggregate ancestral group and the new
forms which have sprung from it by variation, mutation, or adaptation. It
should not be regarded as an isolated unit for purposes of descriptive
botany; indeed, its use in this connection is purely secondary. It is properly
the unit to be used in indicating the primary relationships which are the
result of evolution.
On the basis of their actual behavior in the production of new forms,
species may be distinguished as variable, mutable, or adaptable. The new
form which results from variation is a variant; the product of mutation is a
mutant, and that of adaptation, an ecad. The following examples serve to
illustrate these distinctions. Machaeranthera canescens, judging from the
numerous minute intergrades between its many forms, is a variable species,
i. e., one in which forms are arising by the gradual selection of small
variations. It apparently comprises a large number of variants, M. canescens
aspera, superba, ramosa, viscosa, etc. Onagra lamarckiana is a mutable
species: it comprises many mutants, e. g., Onagra lamarckiana gigas, O. l.
nanella, O. l. lata, etc. Galium boreale is an adaptable species: it possesses
one distinct ecad, Galium boreale hylocolum, which is the shade form of the
species.
187. Methods. The best of all experiments in evolution are those that are
constantly being made in nature. Such experiments are readily discovered
and studied in the case of origin by adaptation; variants present much
greater difficulties, while mutants are very rare under natural conditions.
The method which makes use of these experiments may be termed the
method of natural experiment. The number of ecads which appear naturally
in vegetation is limited, however, and it is consequently very desirable to
produce them artificially, by the method of habitat culture. This method,
Page 197
while involving more labor than the preceding, yields results that are
equally conclusive, and permits the study of practically every species. The
method of control culture, which is carried on in the planthouse, naturally
does not possess the fundamental value of the field methods. It is an
invaluable aid to the latter, however, since it permits the physical factors to
be readily modified and controlled. All these methods are based on the
indispensable use of instruments for the measurement of physical factors.
equally conclusive, and permits the study of practically every species. The
method of control culture, which is carried on in the planthouse, naturally
does not possess the fundamental value of the field methods. It is an
invaluable aid to the latter, however, since it permits the physical factors to
be readily modified and controlled. All these methods are based on the
indispensable use of instruments for the measurement of physical factors.
Page 198
METHOD OF NATURAL EXPERIMENT
188. Selection of species. Species that are producing variants or ecads
are found everywhere in nature; those which give rise to mutants seem,
however, to be extremely rare. Consequently, mutants can not be counted
upon for experimental work, and their study scarcely needs to be
considered. When a mutant is discovered by some fortunate chance, the
mutable species from which it has sprung, and related species as well,
should be subjected to the most critical surveillance, in the hope that new
mutants will occur or the original one reappear. On account of the
suddenness with which they appear, mutants do not lend themselves readily
to natural experiment, and after they have once been discovered, inquiry
into the causes and course of mutation is practicable only by means of
habitat and control cultures. Among variable species, those are most
promising that show a wide range of variation and are found in abundance
over extensive areas. A species which occurs in widely separated, or more
or less isolated areas, furnishes especially favorable material for
investigation, since distance or physical barriers partly eliminate the
leveling due to constant cross-fertilization. The individuals or groups which
show appreciable departure from the type are marked and observed
critically from year to year. The direction of the variation and the rapidity
with which small changes are accumulated can best be determined by
biometrical methods. Representative individuals of the species and each of
its variants should likewise be selected from year to year. After being
photographed, these are preserved as exsiccati, and with the photographs
constitute a complete graphic record of the course of variation. When the
latter is made evident in structural feature also, histological slides are an
invaluable part of the record.
Polydemic species are by far the best and most frequent of all natural
experiments. In addition to plants that are strictly polydemic, i. e., grow in
two or more distinct habitats, there are a large number which occur in
physically different parts of the same habitat. The recognition of
polydemics is the simplest of tasks. As a rule, it requires merely a careful
examination of contiguous formations in order to ascertain the species
common to two or more of them. The latter are naturally most abundant
188. Selection of species. Species that are producing variants or ecads
are found everywhere in nature; those which give rise to mutants seem,
however, to be extremely rare. Consequently, mutants can not be counted
upon for experimental work, and their study scarcely needs to be
considered. When a mutant is discovered by some fortunate chance, the
mutable species from which it has sprung, and related species as well,
should be subjected to the most critical surveillance, in the hope that new
mutants will occur or the original one reappear. On account of the
suddenness with which they appear, mutants do not lend themselves readily
to natural experiment, and after they have once been discovered, inquiry
into the causes and course of mutation is practicable only by means of
habitat and control cultures. Among variable species, those are most
promising that show a wide range of variation and are found in abundance
over extensive areas. A species which occurs in widely separated, or more
or less isolated areas, furnishes especially favorable material for
investigation, since distance or physical barriers partly eliminate the
leveling due to constant cross-fertilization. The individuals or groups which
show appreciable departure from the type are marked and observed
critically from year to year. The direction of the variation and the rapidity
with which small changes are accumulated can best be determined by
biometrical methods. Representative individuals of the species and each of
its variants should likewise be selected from year to year. After being
photographed, these are preserved as exsiccati, and with the photographs
constitute a complete graphic record of the course of variation. When the
latter is made evident in structural feature also, histological slides are an
invaluable part of the record.
Polydemic species are by far the best and most frequent of all natural
experiments. In addition to plants that are strictly polydemic, i. e., grow in
two or more distinct habitats, there are a large number which occur in
physically different parts of the same habitat. The recognition of
polydemics is the simplest of tasks. As a rule, it requires merely a careful
examination of contiguous formations in order to ascertain the species
common to two or more of them. The latter are naturally most abundant
Page 199
along the ecotones between the habitats, and, as a result, transition areas
and mixed formations are almost inexhaustible sources of ecads. Many
adaptable species are found throughout several formations, however, and
such are experiments of the greatest possible value. Not infrequently
species of the manuals are seen to be ecads, in spite of their systematic
treatment, and to constitute natural experiments that can be readily
followed. Finally, it must be kept in mind that some polydemics are stable,
and do not give rise to ecads by structural adaptation. They not only
constitute extremely interesting experiments in themselves, but they should
also be very carefully followed year by year, since it seems probable that
the responses are merely latent, and that they will appear suddenly in the
form of mutants. In natural experiments it is sometimes difficult to
distinguish which form is the ecad and which the original form of the
species. As a rule, however, this point can be determined by the relative
abundance and the distribution, but in cases of serious doubt, it is necessary
to appeal to experimental cultures.
Although habitats differ more or less with respect to all their factors, the
study of polydemics needs to take into account only the direct factors,
water-content, humidity, and light. Humidity as a highly variable factor
plays a secondary part, and in consequence the search for ecads may be
entirely confined to those habitats that show efficient differences in the
amount of water-content or of light. Temperature, wind, etc., do not produce
ecads, and may be ignored, except in so far as they affect the direct factors.
Complexes of factors, such as altitude, slope, and exposure, are likewise
effective only through the action of the component simple factors upon
water and light. The influence of biotic factors is so remote as to be
negligible, especially in view of the fact that ecads are necessarily favorable
adaptations, and are in consequence little subject to selective agencies. The
essential test of a habitat is the production of a distinguishable ecad, but a
knowledge of the water-content and light values of the habitats under
examination is a material aid, since a minute search of each formation is
necessary to reveal all the ecads. It is evident that habitats or areas that do
not show efficient differences of water or light will contain no ecads of their
common species, and also that extreme differences in the amount of either
of these two factors will preclude origin by adaptation to a large degree, on
account of the need for profound readjustment. The general rule followed
by most polydemics is that sun species will give rise to shade forms, and
and mixed formations are almost inexhaustible sources of ecads. Many
adaptable species are found throughout several formations, however, and
such are experiments of the greatest possible value. Not infrequently
species of the manuals are seen to be ecads, in spite of their systematic
treatment, and to constitute natural experiments that can be readily
followed. Finally, it must be kept in mind that some polydemics are stable,
and do not give rise to ecads by structural adaptation. They not only
constitute extremely interesting experiments in themselves, but they should
also be very carefully followed year by year, since it seems probable that
the responses are merely latent, and that they will appear suddenly in the
form of mutants. In natural experiments it is sometimes difficult to
distinguish which form is the ecad and which the original form of the
species. As a rule, however, this point can be determined by the relative
abundance and the distribution, but in cases of serious doubt, it is necessary
to appeal to experimental cultures.
Although habitats differ more or less with respect to all their factors, the
study of polydemics needs to take into account only the direct factors,
water-content, humidity, and light. Humidity as a highly variable factor
plays a secondary part, and in consequence the search for ecads may be
entirely confined to those habitats that show efficient differences in the
amount of water-content or of light. Temperature, wind, etc., do not produce
ecads, and may be ignored, except in so far as they affect the direct factors.
Complexes of factors, such as altitude, slope, and exposure, are likewise
effective only through the action of the component simple factors upon
water and light. The influence of biotic factors is so remote as to be
negligible, especially in view of the fact that ecads are necessarily favorable
adaptations, and are in consequence little subject to selective agencies. The
essential test of a habitat is the production of a distinguishable ecad, but a
knowledge of the water-content and light values of the habitats under
examination is a material aid, since a minute search of each formation is
necessary to reveal all the ecads. It is evident that habitats or areas that do
not show efficient differences of water or light will contain no ecads of their
common species, and also that extreme differences in the amount of either
of these two factors will preclude origin by adaptation to a large degree, on
account of the need for profound readjustment. The general rule followed
by most polydemics is that sun species will give rise to shade forms, and
Page 200
vice versa, and that xerophytes will produce forms of hydrophytic tendency,
or the converse, when the areas concerned are not too remote, and the water
or light differences are efficient, but not inhibitive. Some species are
capable of developing naturally two series of ecads, one in response to
light, the other to water-content, but they, unfortunately, have been found to
be rare. Greatly diversified regions, such as the Rocky mountains, in which
alternation is a peculiarly striking feature of the vegetation, are especially
favorable to the production of ecads, and hence for the study of natural
experiments in origin by adaptation.
189. Determination of factors. For the critical investigation of the origin
of new forms, an exact knowledge of the factors of the habitat, both
physical and biotic, is imperative. In the case of variable species, these
factors determine what variations are of advantage, and thereby the
direction in which the species can develop. They are the agents of selection.
With mutants, the factors of the habitat are apparently neither causative nor
selective, though it seems probable that further study of mutants will show
an essential connection between mutant and factor. In any event, the
persistence of a mutant in nature, and its corresponding ability to initiate
new lines of development, is as much dependent upon the selection exerted
by physical and biotic factors as is the origin of variants. Physical factors
are causative agents in the production of ecads, as has been shown at length
elsewhere. The form and structure of the ecad are the ultimate responses to
the stimuli of light or water-content, and the quantitative determination of
the latter is accordingly of the most fundamental importance. The
measurement of factors has been treated so fully in the preceding chapters
that it is only necessary to point out that the thorough investigation of
habitats by instruments is as indispensable for the study of experimental
evolution as for that of the development and structure of the formation.
Furthermore, it is evident that a knowledge of physical factors is as
imperative for habitat and control cultures as for the method of natural
experiment. In the latter, however, the biotic factors demand unusual
attention, since pollination, isolation, etc., are often decisive factors in
origin by variation and in the persistence of mutants.
Measurements of adjustment, i. e., functional response to the direct factor
concerned, are extremely valuable, but not altogether indispensable to
research in experimental evolution. This is due to the fact that a knowledge
of adjustment is important in tracing the origin of new forms only when
or the converse, when the areas concerned are not too remote, and the water
or light differences are efficient, but not inhibitive. Some species are
capable of developing naturally two series of ecads, one in response to
light, the other to water-content, but they, unfortunately, have been found to
be rare. Greatly diversified regions, such as the Rocky mountains, in which
alternation is a peculiarly striking feature of the vegetation, are especially
favorable to the production of ecads, and hence for the study of natural
experiments in origin by adaptation.
189. Determination of factors. For the critical investigation of the origin
of new forms, an exact knowledge of the factors of the habitat, both
physical and biotic, is imperative. In the case of variable species, these
factors determine what variations are of advantage, and thereby the
direction in which the species can develop. They are the agents of selection.
With mutants, the factors of the habitat are apparently neither causative nor
selective, though it seems probable that further study of mutants will show
an essential connection between mutant and factor. In any event, the
persistence of a mutant in nature, and its corresponding ability to initiate
new lines of development, is as much dependent upon the selection exerted
by physical and biotic factors as is the origin of variants. Physical factors
are causative agents in the production of ecads, as has been shown at length
elsewhere. The form and structure of the ecad are the ultimate responses to
the stimuli of light or water-content, and the quantitative determination of
the latter is accordingly of the most fundamental importance. The
measurement of factors has been treated so fully in the preceding chapters
that it is only necessary to point out that the thorough investigation of
habitats by instruments is as indispensable for the study of experimental
evolution as for that of the development and structure of the formation.
Furthermore, it is evident that a knowledge of physical factors is as
imperative for habitat and control cultures as for the method of natural
experiment. In the latter, however, the biotic factors demand unusual
attention, since pollination, isolation, etc., are often decisive factors in
origin by variation and in the persistence of mutants.
Measurements of adjustment, i. e., functional response to the direct factor
concerned, are extremely valuable, but not altogether indispensable to
research in experimental evolution. This is due to the fact that a knowledge
of adjustment is important in tracing the origin of new forms only when
Page 201
adjustment is followed by adaptation, and in all such cases the ratio
between the two processes seems to be more or less constant. In the present
rudimentary development of the subject, however, it is very desirable to
make use of all methods of measuring functional responses to water and
light that are practicable in the field. Certain methods that are difficult of
application in nature may be used to advantage in control cultures, and the
results thus secured can be used to interpret those obtained from natural
experiments and field cultures.
190. Method of record. As suggested elsewhere, there are four important
kinds of records, which should be made for natural experiments, and
likewise for habitat and control cultures. These are exsiccati, photographs,
biometrical formulae and curves, and histological sections. These serve not
merely as records of what has taken place, but they also make it possible to
trace the course of evolution through a long period with an accuracy
otherwise impossible, and even to foreshadow the changes which will occur
in the future. The possibility of doing this depends primarily upon the
completeness of the record, and for this reason the four methods indicated
should be used conjointly. In the case of ecads and mutants, exsiccati,
photographs, and sections are the most valuable, and in the majority of
cases are sufficient, since both ecads and mutants bear a more distinctive
impress than variants do. On the other hand, since variations are more
minute, the determination of the mean and extreme of variation by
biometrical methods is almost a prerequisite to the use of the other three
methods, which must necessarily be applied to representative individuals.
Exsiccati and photographs are made in the usual way for plants, but it is
an advantage to photograph each ancestral form alongside of its proper
ecads, mutants, or variants, in addition to making detail pictures of each
form and of the organs which show modification. In the collection of
material for histological sections, which deal primarily with the leaf or with
stems in the case of plants with reduced leaves, a few simple precautions
have been found necessary. Whenever possible, material should be killed
where it is collected, since in this way the chloroplasts are fixed in their
normal position. In case leaves that can not be replaced easily have become
wilted, an immersion of 5–6 hours in water will make it possible to kill
them without shrinkage. In selecting leaves, great pains must be taken to
collect only mature leaves. When the plants have a basal rosette, or distinct
radical leaves, mature leaves are taken from both stem and base. In all cases
between the two processes seems to be more or less constant. In the present
rudimentary development of the subject, however, it is very desirable to
make use of all methods of measuring functional responses to water and
light that are practicable in the field. Certain methods that are difficult of
application in nature may be used to advantage in control cultures, and the
results thus secured can be used to interpret those obtained from natural
experiments and field cultures.
190. Method of record. As suggested elsewhere, there are four important
kinds of records, which should be made for natural experiments, and
likewise for habitat and control cultures. These are exsiccati, photographs,
biometrical formulae and curves, and histological sections. These serve not
merely as records of what has taken place, but they also make it possible to
trace the course of evolution through a long period with an accuracy
otherwise impossible, and even to foreshadow the changes which will occur
in the future. The possibility of doing this depends primarily upon the
completeness of the record, and for this reason the four methods indicated
should be used conjointly. In the case of ecads and mutants, exsiccati,
photographs, and sections are the most valuable, and in the majority of
cases are sufficient, since both ecads and mutants bear a more distinctive
impress than variants do. On the other hand, since variations are more
minute, the determination of the mean and extreme of variation by
biometrical methods is almost a prerequisite to the use of the other three
methods, which must necessarily be applied to representative individuals.
Exsiccati and photographs are made in the usual way for plants, but it is
an advantage to photograph each ancestral form alongside of its proper
ecads, mutants, or variants, in addition to making detail pictures of each
form and of the organs which show modification. In the collection of
material for histological sections, which deal primarily with the leaf or with
stems in the case of plants with reduced leaves, a few simple precautions
have been found necessary. Whenever possible, material should be killed
where it is collected, since in this way the chloroplasts are fixed in their
normal position. In case leaves that can not be replaced easily have become
wilted, an immersion of 5–6 hours in water will make it possible to kill
them without shrinkage. In selecting leaves, great pains must be taken to
collect only mature leaves. When the plants have a basal rosette, or distinct
radical leaves, mature leaves are taken from both stem and base. In all cases
Page 202
where the two surfaces of the leaf can not be readily distinguished, the
upper one is clearly marked.
upper one is clearly marked.
Page 203
Page 204
METHOD OF HABITAT CULTURES
191. Scope and advantages. By means of experiments actually made in
the field, practically every species that is capable of modification can be
made to produce new forms, the origin of which can be traced in the
manner already indicated. Field experiments of this sort are especially
favorable to the production of ecads from adaptable species. No attempt has
yet been made to apply it to mutable or variable species, but its ultimate
application to these does not seem at all impossible. The chief advantage of
the method of habitat cultures is seen in the great range of choice in
selecting the plant for experiment, and the habitat or area in which the
experiment is carried out. A polydemic species which already has one or
more ecads can be extended to a number of different habitats of known
value, and a complete series of ecads obtained, based either upon water-
content, or light, or upon both. On the other hand, an endemic species, or
one brought from a remote flora, can be placed in as many habitats as
desired, and the appearance of ecads followed in each. Frequently, results of
much value are obtained in a diversified habitat by growing its most plastic
species in those areas which show the greatest differences in water-content
or light intensity. Habitat cultures give results which are practically as
perfect as those obtained from natural experiments, since the course of
adaptation in no wise depends upon whether the agent by which the seed or
propagule is carried into the new habitat is natural or artificial. Cultures of
this kind further possess the distinct advantage of permitting more or less
modification of the physical factors themselves. However, when it is
desirable to have the factors under as complete control as possible, it is
necessary to use the method of control cultures in the planthouse.
192. Methods. All field experiments in evolution are based upon a
change of habitat. The latter is accomplished by the modification of the
habitat itself, or by the transfer of the species to one or more different
habitats, or to different areas of the same habitat. In both cases the choice of
habitats is made upon the basis of efficient differences of water-content or
light. Saline situations do not constitute an exception, since the chresard is
really the effective stimulus. Cultures at different altitudes, which afford
striking results, appear to concern several factors, but in the final analysis,
191. Scope and advantages. By means of experiments actually made in
the field, practically every species that is capable of modification can be
made to produce new forms, the origin of which can be traced in the
manner already indicated. Field experiments of this sort are especially
favorable to the production of ecads from adaptable species. No attempt has
yet been made to apply it to mutable or variable species, but its ultimate
application to these does not seem at all impossible. The chief advantage of
the method of habitat cultures is seen in the great range of choice in
selecting the plant for experiment, and the habitat or area in which the
experiment is carried out. A polydemic species which already has one or
more ecads can be extended to a number of different habitats of known
value, and a complete series of ecads obtained, based either upon water-
content, or light, or upon both. On the other hand, an endemic species, or
one brought from a remote flora, can be placed in as many habitats as
desired, and the appearance of ecads followed in each. Frequently, results of
much value are obtained in a diversified habitat by growing its most plastic
species in those areas which show the greatest differences in water-content
or light intensity. Habitat cultures give results which are practically as
perfect as those obtained from natural experiments, since the course of
adaptation in no wise depends upon whether the agent by which the seed or
propagule is carried into the new habitat is natural or artificial. Cultures of
this kind further possess the distinct advantage of permitting more or less
modification of the physical factors themselves. However, when it is
desirable to have the factors under as complete control as possible, it is
necessary to use the method of control cultures in the planthouse.
192. Methods. All field experiments in evolution are based upon a
change of habitat. The latter is accomplished by the modification of the
habitat itself, or by the transfer of the species to one or more different
habitats, or to different areas of the same habitat. In both cases the choice of
habitats is made upon the basis of efficient differences of water-content or
light. Saline situations do not constitute an exception, since the chresard is
really the effective stimulus. Cultures at different altitudes, which afford
striking results, appear to concern several factors, but in the final analysis,
Page 205
water-content and humidity are alone found to be really formative. Cultures
may furthermore be distinguished as simple or reciprocal. Simple cultures
are those in which a species is transferred to one or more habitats, or in
which a habitat is modified in one or more ways. Reciprocal cultures are
possible only with polydemic species, or with endemics after ecads have
been produced by experiment. Modification or transfer is made in the usual
way, but reciprocally, i. e., the original form is transferred to the habitat of
the ecad, and the latter to the habitat of the former; or the shade in which
some individuals of the ecad are growing may be destroyed, and at the same
time individuals of the type may be shaded. Both transfer and modification
may be applied to the same species, but since the same measured change of
factor can be obtained in either way, the use of both is undesirable, with the
exception of the rare cases where they serve as checks upon each other. The
transfer of a seed or plant is so much simpler and more convenient that this
method is the one regularly used. It sometimes happens, however, that a
change of water-content or light intensity is readily and conveniently made,
and is desirable for other reasons.
It is evident that both transfer and modification require that the factor
records of the various habitats or areas be as full as possible, at least so far
as water-content, humidity, and light are concerned. In the case of the areas
that are to be modified, these factors are determined before the change is
made. Afterward they are read from time to time during the growing season,
and are also checked by readings made near at hand in the unmodified
formation. The readings made in the beginning should correspond closely to
the check readings, but in case of disagreement the latter are to be taken as
conclusive.
193. Transfer. After the species to be used for experiment has been
chosen, the various habitats or areas selected, and the direct factors
measured by instruments, the actual transfer of the individuals is made by
means of seeds, preferably in autumn, though the results are practically the
same if seeds are kept over the winter and planted at the opening of spring.
The natural method is to scatter the seeds in the place selected, as though
they had been carried by the usual agents of migration. The mortality is
usually great in such case, however, and the chances of success are
increased by actually planting the seeds. This is the method which has been
used in making cultures of species of the European Alps on the summit of
Mount Garfield in the Rocky mountains. The number of seeds used is
may furthermore be distinguished as simple or reciprocal. Simple cultures
are those in which a species is transferred to one or more habitats, or in
which a habitat is modified in one or more ways. Reciprocal cultures are
possible only with polydemic species, or with endemics after ecads have
been produced by experiment. Modification or transfer is made in the usual
way, but reciprocally, i. e., the original form is transferred to the habitat of
the ecad, and the latter to the habitat of the former; or the shade in which
some individuals of the ecad are growing may be destroyed, and at the same
time individuals of the type may be shaded. Both transfer and modification
may be applied to the same species, but since the same measured change of
factor can be obtained in either way, the use of both is undesirable, with the
exception of the rare cases where they serve as checks upon each other. The
transfer of a seed or plant is so much simpler and more convenient that this
method is the one regularly used. It sometimes happens, however, that a
change of water-content or light intensity is readily and conveniently made,
and is desirable for other reasons.
It is evident that both transfer and modification require that the factor
records of the various habitats or areas be as full as possible, at least so far
as water-content, humidity, and light are concerned. In the case of the areas
that are to be modified, these factors are determined before the change is
made. Afterward they are read from time to time during the growing season,
and are also checked by readings made near at hand in the unmodified
formation. The readings made in the beginning should correspond closely to
the check readings, but in case of disagreement the latter are to be taken as
conclusive.
193. Transfer. After the species to be used for experiment has been
chosen, the various habitats or areas selected, and the direct factors
measured by instruments, the actual transfer of the individuals is made by
means of seeds, preferably in autumn, though the results are practically the
same if seeds are kept over the winter and planted at the opening of spring.
The natural method is to scatter the seeds in the place selected, as though
they had been carried by the usual agents of migration. The mortality is
usually great in such case, however, and the chances of success are
increased by actually planting the seeds. This is the method which has been
used in making cultures of species of the European Alps on the summit of
Mount Garfield in the Rocky mountains. The number of seeds used is
Page 206
recorded in order to obtain some estimate of germination and competition.
While the use of the seed or disseminule possesses the great advantage of
making the experiment essentially a natural one, the transfer of rosettes,
seedlings, or young plants makes the results more certain, and consequently
saves time, even though the actual transfer is somewhat more difficult. It is
hardly necessary to point out that the removal of the plant should be made
with the greatest care. The best success is obtained by making the transfer
on cloudy or rainy days, and when shade plants are to be placed in sunny
situations, they should be transplanted late in the afternoon. When the task
of carrying them is not too great, it is a distinct advantage to move a
number of individuals in the same block of earth. The transfer of mature
plants is inadvisable, except for those perennials which can not readily be
secured in an early stage. This naturally does not apply to woody plants,
evergreen herbs, mosses and lichens; the last two may be transferred at any
time with satisfactory results. Each culture is carefully marked with stakes,
and definitely located by means of landmarks.
While the use of the seed or disseminule possesses the great advantage of
making the experiment essentially a natural one, the transfer of rosettes,
seedlings, or young plants makes the results more certain, and consequently
saves time, even though the actual transfer is somewhat more difficult. It is
hardly necessary to point out that the removal of the plant should be made
with the greatest care. The best success is obtained by making the transfer
on cloudy or rainy days, and when shade plants are to be placed in sunny
situations, they should be transplanted late in the afternoon. When the task
of carrying them is not too great, it is a distinct advantage to move a
number of individuals in the same block of earth. The transfer of mature
plants is inadvisable, except for those perennials which can not readily be
secured in an early stage. This naturally does not apply to woody plants,
evergreen herbs, mosses and lichens; the last two may be transferred at any
time with satisfactory results. Each culture is carefully marked with stakes,
and definitely located by means of landmarks.
Page 207
Fig. 46. Series for producing hydrophytic forms under control: 1, amphibious; 2, floating; 3,
competition; 4, submerged.
Reciprocal transfers may be made by means of seed or plant. Since the
experiment is a complex one, all the care possible should be taken to make
sure that the plants become established in the reciprocal situations, and
consequently, it is often advisable to transfer both seeds and plants.
Reciprocal transfer is of paramount value in solving the problem which bog
plants present. A slight modification of the method makes it possible to
obtain experimental evidence of the polyphyletic origin of species in
consequence of adaptation. In an experiment mentioned elsewhere, the
transfer of Kuhnistera purpurea to the area occupied by K. candida, and
vice versa, is designed to show whether one has been derived from the
other. If the two species are moved into an area which contains more water
than that usually occupied by K. purpurea, and less water than is found
competition; 4, submerged.
Reciprocal transfers may be made by means of seed or plant. Since the
experiment is a complex one, all the care possible should be taken to make
sure that the plants become established in the reciprocal situations, and
consequently, it is often advisable to transfer both seeds and plants.
Reciprocal transfer is of paramount value in solving the problem which bog
plants present. A slight modification of the method makes it possible to
obtain experimental evidence of the polyphyletic origin of species in
consequence of adaptation. In an experiment mentioned elsewhere, the
transfer of Kuhnistera purpurea to the area occupied by K. candida, and
vice versa, is designed to show whether one has been derived from the
other. If the two species are moved into an area which contains more water
than that usually occupied by K. purpurea, and less water than is found
Page 208
where K. candida habitually grows, the resulting modifications will throw
much light upon the origin of polyphyletic species. In this connection, it
hardly needs to be pointed out that this simple transfer of a species to
several separated areas of a new habitat may often furnish complete proof
that a new form may arise at different times, and at different places.
194. Modification of the
habitat. Efficient changes in the
habitat are brought about by
increasing or decreasing the water-
content, or by varying the light
intensity between sunshine and the
diffuse light of deep forests.
Humidity can not well be regulated
except in so far as it is connected
with water-content. Since its
effects merge with those of the
latter, its modification is
unnecessary. An increase in water-
content is readily brought about by
irrigation. A stream may be
dammed and its water allowed to
spread over the area to be studied,
Fig. 47. Control ecad of Ranunculus
or the water may be carried to the
sceleratus, holard 10% (50 cc.).
proper place by deflecting the
stream or by digging a canal. The
construction of earth reservoirs makes it possible to obtain almost any per
cent of soil water by varying the size of the reservoir or the height of the
wall or bank. Near a base station, such as Minnehaha, where there is a
simple system of water-works, the experimental area may be watered
whenever desirable by means of a hose. Water-content may be readily
decreased by drainage, or by the deflection of a stream. When such means
are not available, as in the case of extensive marshes, hummocks may be
used or constructed, and the soil blocks containing plants placed upon them.
By the use of sand or gravel, the water-content of mesophytic areas can be
reduced in a similar manner, or by surrounding the plant in situ with either
of these soils which hold little water. In meadows, especially, the addition
of a large quantity of alkaline salts decreases the amount of available water,
much light upon the origin of polyphyletic species. In this connection, it
hardly needs to be pointed out that this simple transfer of a species to
several separated areas of a new habitat may often furnish complete proof
that a new form may arise at different times, and at different places.
194. Modification of the
habitat. Efficient changes in the
habitat are brought about by
increasing or decreasing the water-
content, or by varying the light
intensity between sunshine and the
diffuse light of deep forests.
Humidity can not well be regulated
except in so far as it is connected
with water-content. Since its
effects merge with those of the
latter, its modification is
unnecessary. An increase in water-
content is readily brought about by
irrigation. A stream may be
dammed and its water allowed to
spread over the area to be studied,
Fig. 47. Control ecad of Ranunculus
or the water may be carried to the
sceleratus, holard 10% (50 cc.).
proper place by deflecting the
stream or by digging a canal. The
construction of earth reservoirs makes it possible to obtain almost any per
cent of soil water by varying the size of the reservoir or the height of the
wall or bank. Near a base station, such as Minnehaha, where there is a
simple system of water-works, the experimental area may be watered
whenever desirable by means of a hose. Water-content may be readily
decreased by drainage, or by the deflection of a stream. When such means
are not available, as in the case of extensive marshes, hummocks may be
used or constructed, and the soil blocks containing plants placed upon them.
By the use of sand or gravel, the water-content of mesophytic areas can be
reduced in a similar manner, or by surrounding the plant in situ with either
of these soils which hold little water. In meadows, especially, the addition
of a large quantity of alkaline salts decreases the amount of available water,
Page 209
while the holard may be reduced by denuding the soil about the plants
concerned.
In sunny habitats, the light
intensity is most easily reduced by
means of cloth awnings, which can
be put in place conveniently. It is
not a difficult matter to produce
effective shade by using shrubs or
small trees for this purpose. This
plan is especially advantageous in
habitats too remote to make
frequent visits feasible. When a
shrub or tree is used, the
experiment necessarily requires a
longer time, though this
disadvantage is partly compensated
by the fact that the shelter requires
practically no attention after the
shrub is once established. Forest
plantations furnish excellent Fig. 48. Control ecad of Ranunculus
examples of this kind of sceleratus, holard 40% (200 cc.).
experiment. On the other hand,
clearings afford the only examples
of habitats modified in such manner as to increase the light. In nature, the
diffuse light in which shade plants grow is due to the presence of tall plants,
chiefly shrubs and trees, and an increase in the light intensity is possible
only through the thinning-out or removal of the plant screen. This is a task
of considerable magnitude in forests, but it can be readily accomplished in
thickets and at the edges of woodlands. It is quite practicable to establish a
series of awnings or clearings of various light values, but the labor required
is hardly worth while when it is recalled that the method of transfer makes
it possible to take advantage of the various intensities already found in
nature.
concerned.
In sunny habitats, the light
intensity is most easily reduced by
means of cloth awnings, which can
be put in place conveniently. It is
not a difficult matter to produce
effective shade by using shrubs or
small trees for this purpose. This
plan is especially advantageous in
habitats too remote to make
frequent visits feasible. When a
shrub or tree is used, the
experiment necessarily requires a
longer time, though this
disadvantage is partly compensated
by the fact that the shelter requires
practically no attention after the
shrub is once established. Forest
plantations furnish excellent Fig. 48. Control ecad of Ranunculus
examples of this kind of sceleratus, holard 40% (200 cc.).
experiment. On the other hand,
clearings afford the only examples
of habitats modified in such manner as to increase the light. In nature, the
diffuse light in which shade plants grow is due to the presence of tall plants,
chiefly shrubs and trees, and an increase in the light intensity is possible
only through the thinning-out or removal of the plant screen. This is a task
of considerable magnitude in forests, but it can be readily accomplished in
thickets and at the edges of woodlands. It is quite practicable to establish a
series of awnings or clearings of various light values, but the labor required
is hardly worth while when it is recalled that the method of transfer makes
it possible to take advantage of the various intensities already found in
nature.
Page 210
METHOD OF CONTROL CULTURES
195. Scope and procedure. Control experiments are necessarily carried
on in the planthouse, since factors can be controlled in the field only with
great difficulty. Their greatest value is in connection with experiments that
are being carried on in the habitat, but they also constitute an invaluable
means of independent research, since it is not at all difficult to approximate
the conditions of a habitat, especially with reference to water-content and
light. The essential feature of the method is that the less important factors
are equalized as far as possible, while the direct factors, water-content and
light, are under the complete control of the investigator. By the equalization
of humidity and temperature is meant experimentation in which all the
plants of each experiment are subjected to the same amounts of these
factors. It is a matter of no importance whatever whether the humidity and
temperature are constant or variable. In the case of soil, which is not a
variable, it naturally happens that the plants are placed once for all in the
same soil mixture. Batteries consisting of thermograph and psychrograph
have been kept in the different control houses, but although used at first to
give some idea of the hourly and daily fluctuations of temperature and
humidity, they have slight bearing upon the evolution of new forms under
control. For use in connection with supplementary experiments in
adjustment and adaptation, the batteries have proved to be indispensable.
Control experiments are regularly made in series which are planned with
reference to as many modifications as the efficient difference of the factor
and the plasticity of the species concerned permit.
196. Water-content series. An account of the experiments which have
been carried on for four generations with Ranunculus sceleratus will serve
to show the application of culture methods to the origin of new forms in
response to varying water-content. This species was chosen because it
grows readily in the planthouse, is plastic, and, since it is naturally
amphibious, permits of much modification in both directions. The smallest
amount of water per day under which the seedlings would grow was found
to be 25 cc. This was taken as one extreme for the series, and deep water in
which the plant could be submerged as the other. An arbitrary series was
tentatively made as follows: 25 cc., 50 cc., 100 c., 150 cc., 200 cc., mud,
195. Scope and procedure. Control experiments are necessarily carried
on in the planthouse, since factors can be controlled in the field only with
great difficulty. Their greatest value is in connection with experiments that
are being carried on in the habitat, but they also constitute an invaluable
means of independent research, since it is not at all difficult to approximate
the conditions of a habitat, especially with reference to water-content and
light. The essential feature of the method is that the less important factors
are equalized as far as possible, while the direct factors, water-content and
light, are under the complete control of the investigator. By the equalization
of humidity and temperature is meant experimentation in which all the
plants of each experiment are subjected to the same amounts of these
factors. It is a matter of no importance whatever whether the humidity and
temperature are constant or variable. In the case of soil, which is not a
variable, it naturally happens that the plants are placed once for all in the
same soil mixture. Batteries consisting of thermograph and psychrograph
have been kept in the different control houses, but although used at first to
give some idea of the hourly and daily fluctuations of temperature and
humidity, they have slight bearing upon the evolution of new forms under
control. For use in connection with supplementary experiments in
adjustment and adaptation, the batteries have proved to be indispensable.
Control experiments are regularly made in series which are planned with
reference to as many modifications as the efficient difference of the factor
and the plasticity of the species concerned permit.
196. Water-content series. An account of the experiments which have
been carried on for four generations with Ranunculus sceleratus will serve
to show the application of culture methods to the origin of new forms in
response to varying water-content. This species was chosen because it
grows readily in the planthouse, is plastic, and, since it is naturally
amphibious, permits of much modification in both directions. The smallest
amount of water per day under which the seedlings would grow was found
to be 25 cc. This was taken as one extreme for the series, and deep water in
which the plant could be submerged as the other. An arbitrary series was
tentatively made as follows: 25 cc., 50 cc., 100 c., 150 cc., 200 cc., mud,
Page 211
shallow water, and deep water. Further study justified these divisions, since
the first six gave efficient differences in water-content, and the resulting
forms all showed differences of structure as well as of growth and form.
Seedlings of the same age, and as nearly alike as possible, were
transplanted to large pots of which there were four for each of the first six;
they were placed in half-barrels for mud and floating forms, and in a barrel
for submerged forms. After a few days, when they had become well
established, the plants in the pots were watered in the amounts indicated, as
often as was necessary to keep the most xerophytic form alive; the soil for
the mud form was kept covered with a thin film of water; the leaves of the
form in shallow water were kept floating on the surface, and those of the
last form submerged just below the surface. The water in which the
submerged form grew was aerated by means of a spigot near the bottom of
the barrel. From time to time water-content determinations were made of
the soil in the pots until it was definitely ascertained that the holard was
practically constant. The nine new forms obtained by adaptation showed
striking differences in vigor and growth, as may be seen from the figures. In
all cases, these were accompanied by distinct and often striking differences
in the number and position of the stomata, the amount of sponge and
palisade tissues, and the development of air passages. Photographs were
made of a typical plant of each form, and the different leaf structures were
preserved in permanent mounts. The xerophytic and the submerged form
were unable to produce flowers, and it was necessary to develop them anew
in each generation. The other forms fruited abundantly, and the succeeding
generations of each form were produced from plants which had grown the
year before in the same conditions. In addition to the development of a
series of new water-content forms, this experiment was begun in the hope
of determining whether the modifications of a plastic species tend to
become fixed if each new form is grown constantly under the same
conditions. A period of four years is too short, however, to throw much light
upon this problem.
Helianthus annuus has been used for other series of experiments, in
which alkaline salts or different soils are employed to vary the water-
content. These are more complex and hence are not as satisfactory as the
series described above, but they are valuable for the light they throw upon
the behavior of plants in similar conditions in nature. In the case of soil,
however, the adaptation may be referred to water-content alone, if
the first six gave efficient differences in water-content, and the resulting
forms all showed differences of structure as well as of growth and form.
Seedlings of the same age, and as nearly alike as possible, were
transplanted to large pots of which there were four for each of the first six;
they were placed in half-barrels for mud and floating forms, and in a barrel
for submerged forms. After a few days, when they had become well
established, the plants in the pots were watered in the amounts indicated, as
often as was necessary to keep the most xerophytic form alive; the soil for
the mud form was kept covered with a thin film of water; the leaves of the
form in shallow water were kept floating on the surface, and those of the
last form submerged just below the surface. The water in which the
submerged form grew was aerated by means of a spigot near the bottom of
the barrel. From time to time water-content determinations were made of
the soil in the pots until it was definitely ascertained that the holard was
practically constant. The nine new forms obtained by adaptation showed
striking differences in vigor and growth, as may be seen from the figures. In
all cases, these were accompanied by distinct and often striking differences
in the number and position of the stomata, the amount of sponge and
palisade tissues, and the development of air passages. Photographs were
made of a typical plant of each form, and the different leaf structures were
preserved in permanent mounts. The xerophytic and the submerged form
were unable to produce flowers, and it was necessary to develop them anew
in each generation. The other forms fruited abundantly, and the succeeding
generations of each form were produced from plants which had grown the
year before in the same conditions. In addition to the development of a
series of new water-content forms, this experiment was begun in the hope
of determining whether the modifications of a plastic species tend to
become fixed if each new form is grown constantly under the same
conditions. A period of four years is too short, however, to throw much light
upon this problem.
Helianthus annuus has been used for other series of experiments, in
which alkaline salts or different soils are employed to vary the water-
content. These are more complex and hence are not as satisfactory as the
series described above, but they are valuable for the light they throw upon
the behavior of plants in similar conditions in nature. In the case of soil,
however, the adaptation may be referred to water-content alone, if
Page 212
thoroughly leached sands and
gravels are used, so that the
difference is solely one of water-
retaining power.
197. Light series. Cloth tents
have been found the most
satisfactory means of obtaining
different light intensities in the
planthouse. The cloth permits the
air to circulate to a considerable
degree, and in consequence the
equalization of humidity and
temperature is much more
complete than in the glass houses
first employed. The cloth tents, or
shade tents as they are called, are
cubical, each dimension being 1
meter. The series which has been Fig. 49. Floating form of Ranunculus
most used consists of three tents: sceleratus grown under control.
the first is made of cheesecloth and
has a light value of .1; the second
is of thin muslin, and has a value of .04, while the third is made of dark
cambric and the light is reduced to .01. A more desirable series is one with
five tents, which have approximately the following light intensities: .1, .05,
.01, .007, .003. Plants grown in shade tents should be repotted as often as
they will permit in order to increase the aeration of the soil. The amount of
water given them must also be decreased as the shade increases.
Mesophytic species give the best results in shade tents, xerophytes thrive
less well, and amphibious plants do not grow at all except in the brightest
light. Excellent results have been obtained with Helianthus, Taraxacum,
Gaura, and Onagra, while Ranunculus sceleratus is unable to produce
flowers and seeds in a light intensity of .01.
A number of important supplementary experiments have been made in
connection with light tents. These do not result in the production of new
forms, but they throw much light upon it. Plants have been placed in the
shade tents so that certain leaves would be in the sun and others in the
shade. Young leaves have been fixed at various angles with the stem, and
gravels are used, so that the
difference is solely one of water-
retaining power.
197. Light series. Cloth tents
have been found the most
satisfactory means of obtaining
different light intensities in the
planthouse. The cloth permits the
air to circulate to a considerable
degree, and in consequence the
equalization of humidity and
temperature is much more
complete than in the glass houses
first employed. The cloth tents, or
shade tents as they are called, are
cubical, each dimension being 1
meter. The series which has been Fig. 49. Floating form of Ranunculus
most used consists of three tents: sceleratus grown under control.
the first is made of cheesecloth and
has a light value of .1; the second
is of thin muslin, and has a value of .04, while the third is made of dark
cambric and the light is reduced to .01. A more desirable series is one with
five tents, which have approximately the following light intensities: .1, .05,
.01, .007, .003. Plants grown in shade tents should be repotted as often as
they will permit in order to increase the aeration of the soil. The amount of
water given them must also be decreased as the shade increases.
Mesophytic species give the best results in shade tents, xerophytes thrive
less well, and amphibious plants do not grow at all except in the brightest
light. Excellent results have been obtained with Helianthus, Taraxacum,
Gaura, and Onagra, while Ranunculus sceleratus is unable to produce
flowers and seeds in a light intensity of .01.
A number of important supplementary experiments have been made in
connection with light tents. These do not result in the production of new
forms, but they throw much light upon it. Plants have been placed in the
shade tents so that certain leaves would be in the sun and others in the
shade. Young leaves have been fixed at various angles with the stem, and
Page 213
they have been revolved 90° or 180° in order to change the relation of their
surfaces. Soils of different colors, e. g., loam and sand, have been used to
determine the effect of light reflected from their surfaces. Shade tents make
it possible to illuminate plants from the top, bottom, or side, and to carry on
a large number of fundamental experiments in adjustment and adaptation.
surfaces. Soils of different colors, e. g., loam and sand, have been used to
determine the effect of light reflected from their surfaces. Shade tents make
it possible to illuminate plants from the top, bottom, or side, and to carry on
a large number of fundamental experiments in adjustment and adaptation.
Page 214
CHAPTER IV. THE PLANT FORMATION
Methods of Investigation and Record
198. The need of exact methods. The use of instruments in the study of
the habitat has made it evident that the loose methods of descriptive
ecology were altogether inadequate to the accurate investigation of the
formation. This feeling has been heightened by the recognition of the fact
that vegetation exhibits both development and structure, and is, in
consequence, open to exact methods of inquiry. In the search for feasible
methods, it was quickly seen that the quadrat, first[20] used for determining
the abundance of species, furnished the key to the problem. Accordingly,
the principle underlying it, viz., that of intimate detailed study and record,
was developed and extended in such a way as to give rise to a number of
methods of precision. These have been applied in the field for several years
with signal success, and they are here described in the conviction that they
constitute a satisfactory system, if not, indeed, the only one for the exact
study of formations.
There has been a growing appreciation of the fact that the superficial
methods of descriptive ecology made it impossible to build upon such a
foundation, and they, indeed, were making actual progress in the field of
ecology more and more difficult. Ecologists have now begun to see clearly
that precise methods are as indispensable in the habitat as they are to the
study of the structure and modification of the plant. For some reason,
however, they have been slow to perceive that accuracy in the investigation
of the cause, the habitat, is a fruitless task unless it be followed by
corresponding exactness in the study of the effect, the formation. After
having urged the fundamental necessity of instrumental methods, for six or
seven years, both in season and out of season, the writer does not feel called
upon to further plead the cause of the quadrat. The final acceptance of the
Methods of Investigation and Record
198. The need of exact methods. The use of instruments in the study of
the habitat has made it evident that the loose methods of descriptive
ecology were altogether inadequate to the accurate investigation of the
formation. This feeling has been heightened by the recognition of the fact
that vegetation exhibits both development and structure, and is, in
consequence, open to exact methods of inquiry. In the search for feasible
methods, it was quickly seen that the quadrat, first[20] used for determining
the abundance of species, furnished the key to the problem. Accordingly,
the principle underlying it, viz., that of intimate detailed study and record,
was developed and extended in such a way as to give rise to a number of
methods of precision. These have been applied in the field for several years
with signal success, and they are here described in the conviction that they
constitute a satisfactory system, if not, indeed, the only one for the exact
study of formations.
There has been a growing appreciation of the fact that the superficial
methods of descriptive ecology made it impossible to build upon such a
foundation, and they, indeed, were making actual progress in the field of
ecology more and more difficult. Ecologists have now begun to see clearly
that precise methods are as indispensable in the habitat as they are to the
study of the structure and modification of the plant. For some reason,
however, they have been slow to perceive that accuracy in the investigation
of the cause, the habitat, is a fruitless task unless it be followed by
corresponding exactness in the study of the effect, the formation. After
having urged the fundamental necessity of instrumental methods, for six or
seven years, both in season and out of season, the writer does not feel called
upon to further plead the cause of the quadrat. The final acceptance of the
Page 215
instrument was inevitable if progress were to be made in the habitat, and it
is just as obvious that the quadrat must be accepted if the study of the
habitat is to bear fruit in the interpretation of the formation. The use of the
quadrat does not mean that the general methods of descriptive ecology are
all to be discarded, whether they have value or not. The statement that
quadrat methods are indispensable signifies merely that they must be used
for research work in the development and structure of vegetation. They are
not necessary in reconnaissance, nor do they displace general methods of
real value. The use of the latter in even a supplementary way will gradually
be discontinued, however, as fields become smaller by reason of increase in
the number of workers, and as the need for precise methods becomes more
universally felt.
The quadrat constitutes the initial concept from which all the methods
have grown. In itself, it has given rise to a variety of quadrats applicable to
the most fundamental problems of vegetation. From it have come, on the
one hand, the migration circle, and on the other, the transect. The latter in
turn has yielded the ecotone chart, and the layer chart. All of these are based
upon direct and detailed contact with vegetation itself, and permit accurate
recording of all the results obtained.
is just as obvious that the quadrat must be accepted if the study of the
habitat is to bear fruit in the interpretation of the formation. The use of the
quadrat does not mean that the general methods of descriptive ecology are
all to be discarded, whether they have value or not. The statement that
quadrat methods are indispensable signifies merely that they must be used
for research work in the development and structure of vegetation. They are
not necessary in reconnaissance, nor do they displace general methods of
real value. The use of the latter in even a supplementary way will gradually
be discontinued, however, as fields become smaller by reason of increase in
the number of workers, and as the need for precise methods becomes more
universally felt.
The quadrat constitutes the initial concept from which all the methods
have grown. In itself, it has given rise to a variety of quadrats applicable to
the most fundamental problems of vegetation. From it have come, on the
one hand, the migration circle, and on the other, the transect. The latter in
turn has yielded the ecotone chart, and the layer chart. All of these are based
upon direct and detailed contact with vegetation itself, and permit accurate
recording of all the results obtained.
Page 216
QUADRATS
199. Uses. In its simplest form, the quadrat, as the name implies, is
merely a square area of varying size marked off in a formation for the
purpose of obtaining accurate information as to the number and grouping of
the plants present. As indicated above, it was first used for determining the
abundance of the various species of a formation. This made it possible to
ascertain the relative rank of the species of layers and formations, and
enabled one for the first time to gain some idea of the minute structure of a
bit of vegetation. The results were at once applied to the task of establishing
a numerical basis for abundance, and of working out a new system of
abundance to correspond. The quadrat method was also used to determine
the character of seasonal aspects, and to yield a knowledge of the exact
differences in diverse areas of the same formation. Incidentally, the
determinations of abundance were made the basis of an actual census of
certain alpine formations. This, while it was extremely interesting to find
that a square mile of alpine meadow contained approximately
1,500,000,000 plants, was confessedly destitute of ecological value. The
most important applications of the quadrat idea were made by Clements[21]
in the chart, the permanent and the denuded quadrats. The development of
these was due to the fact that zones or formations permit of comparison
upon floristic as well as physical grounds, and that a detailed record of their
structure is necessary for this purpose. Similar comparisons are necessary
for the consocies, zones, and patches of the same formation, and the quadrat
becomes an indispensable means for studying alternation and zonation. For
the investigation of invasion year by year, and especially for succession, the
method of permanent quadrats is imperative, and the denuded quadrat an
invaluable aid. Changes, which would otherwise be incompletely observed
and imperfectly recorded, are followed in the minutest detail and recorded
with perfect accuracy.
200. Possible objections. The use of the quadrat has led to the criticism
that it is needlessly detailed and thorough, and that, after all, the space
covered is but a minute part of the entire formation. The first objection is
one that has also been urged against the use of instruments of precision in
the habitat. It is always brought forward by those who have not used
199. Uses. In its simplest form, the quadrat, as the name implies, is
merely a square area of varying size marked off in a formation for the
purpose of obtaining accurate information as to the number and grouping of
the plants present. As indicated above, it was first used for determining the
abundance of the various species of a formation. This made it possible to
ascertain the relative rank of the species of layers and formations, and
enabled one for the first time to gain some idea of the minute structure of a
bit of vegetation. The results were at once applied to the task of establishing
a numerical basis for abundance, and of working out a new system of
abundance to correspond. The quadrat method was also used to determine
the character of seasonal aspects, and to yield a knowledge of the exact
differences in diverse areas of the same formation. Incidentally, the
determinations of abundance were made the basis of an actual census of
certain alpine formations. This, while it was extremely interesting to find
that a square mile of alpine meadow contained approximately
1,500,000,000 plants, was confessedly destitute of ecological value. The
most important applications of the quadrat idea were made by Clements[21]
in the chart, the permanent and the denuded quadrats. The development of
these was due to the fact that zones or formations permit of comparison
upon floristic as well as physical grounds, and that a detailed record of their
structure is necessary for this purpose. Similar comparisons are necessary
for the consocies, zones, and patches of the same formation, and the quadrat
becomes an indispensable means for studying alternation and zonation. For
the investigation of invasion year by year, and especially for succession, the
method of permanent quadrats is imperative, and the denuded quadrat an
invaluable aid. Changes, which would otherwise be incompletely observed
and imperfectly recorded, are followed in the minutest detail and recorded
with perfect accuracy.
200. Possible objections. The use of the quadrat has led to the criticism
that it is needlessly detailed and thorough, and that, after all, the space
covered is but a minute part of the entire formation. The first objection is
one that has also been urged against the use of instruments of precision in
the habitat. It is always brought forward by those who have not used
Page 217
instruments, and as witnesses they are of necessity incompetent. No one
who is familiar with the instrument or the quadrat by actual practice has felt
that the methods based upon them were too thorough. In no case has the
writer ever listed or mapped a quadrat without discovering some new fact
or relation, or clearing up an old question. It can not be denied that quadrat
methods require both time and patience, but this is true of any kind of
research work that is at all worth while. Every ecologist, moreover, that has
the interests of his field at heart and deprecates the present slipshod work,
will appreciate the necessity of methods which seem like drudgery to the
mere dabbler.
The second objection, that the quadrat is at best but a small bit of the area
under investigation, seems at first to be a valid one. It can not be gainsaid
that the actual space studied is insignificant as compared with the whole
formation; still, it must be obvious that even a single quadrat can add at
least some facts of value, which can never be obtained by the best of
general methods. Furthermore, if the formation be an actual and not an
imaginary one, a single quadrat will be in some measure representative. In
the more homogeneous ones, it will have much the same value that a type
specimen bears to the species established upon it. In formations which are
less uniform, its value is correspondingly reduced, so that in formations
which show marked zones, consocies, or patches, it becomes necessary to
locate a quadrat in each. In the matter of representation alone, the graphic
method of the quadrat map with its close-focus detail photograph, is far
superior to anything that can be obtained by the ordinary description and
photograph. Finally, the scientific study and recording of succession, and
particularly of competition, is an impossibility without the aid of the
permanent and denuded quadrat. The stoutest champion of the practice of
walking through a formation, and jotting down impressions, can not avoid
their use if he would attack these problems, and, once familiar with the
quadrat, his objections to the drudgery of thoroughness will soon vanish.
Kinds of Quadrats and Their Use
201. Size and kinds. The unit size of quadrat is the meter, and when the
term is used without qualification, it refers to the meter quadrat. To make
them strictly comparable, and exactly divisible, unit quadrats are always
who is familiar with the instrument or the quadrat by actual practice has felt
that the methods based upon them were too thorough. In no case has the
writer ever listed or mapped a quadrat without discovering some new fact
or relation, or clearing up an old question. It can not be denied that quadrat
methods require both time and patience, but this is true of any kind of
research work that is at all worth while. Every ecologist, moreover, that has
the interests of his field at heart and deprecates the present slipshod work,
will appreciate the necessity of methods which seem like drudgery to the
mere dabbler.
The second objection, that the quadrat is at best but a small bit of the area
under investigation, seems at first to be a valid one. It can not be gainsaid
that the actual space studied is insignificant as compared with the whole
formation; still, it must be obvious that even a single quadrat can add at
least some facts of value, which can never be obtained by the best of
general methods. Furthermore, if the formation be an actual and not an
imaginary one, a single quadrat will be in some measure representative. In
the more homogeneous ones, it will have much the same value that a type
specimen bears to the species established upon it. In formations which are
less uniform, its value is correspondingly reduced, so that in formations
which show marked zones, consocies, or patches, it becomes necessary to
locate a quadrat in each. In the matter of representation alone, the graphic
method of the quadrat map with its close-focus detail photograph, is far
superior to anything that can be obtained by the ordinary description and
photograph. Finally, the scientific study and recording of succession, and
particularly of competition, is an impossibility without the aid of the
permanent and denuded quadrat. The stoutest champion of the practice of
walking through a formation, and jotting down impressions, can not avoid
their use if he would attack these problems, and, once familiar with the
quadrat, his objections to the drudgery of thoroughness will soon vanish.
Kinds of Quadrats and Their Use
201. Size and kinds. The unit size of quadrat is the meter, and when the
term is used without qualification, it refers to the meter quadrat. To make
them strictly comparable, and exactly divisible, unit quadrats are always
Page 218
grouped in squares; thus a major quadrat is a square of four units, and a
perquadrat one of sixteen units, or four meters square. Quadrats of greater
size are necessary in woodland and forest, where the rule, however, is that
the woody plants alone are recorded for the whole quadrat, the herbaceous
growth being listed or mapped for but one or two representative units. For
special purposes, quadrats of 3, 5, 6, etc., meters may be used, but they are
much less convenient. Quadrats are further distinguished with respect to
their use. A list quadrat is one in which the plants are merely listed and the
number of individuals of each species indicated. Chart quadrats are those in
which the area concerned is accurately mapped on plotting paper. Both list
and chart quadrats are rendered permanent by careful labeling, so that their
changes can be followed from year to year. The greater value of the chart
causes practically all permanent quadrats to be of this type, and for the
same reason only permanent chart quadrats are converted into denuded
ones.
202. Tapes and stakes. The lines for marking out quadrats are made of
strong white tape, ⅝ inches wide. This is doubled and sewed firmly at both
edges. Under moderate stretching, the tape is carefully marked off into
decimeters, and eyelets 5 mm. in diameter are set in at each end and at the
marks. This can readily be done by any shoemaker at slight expense. The
usual lengths are one and two meters, as these are most frequently used, and
they can also be easily combined to make larger quadrats. The tapes are
slightly longer than one meter in order that the distance between the end
eyelets may be exact. The tapes of the larger forest quadrats should be
divided into lengths of one meter, as these permit ready plotting and also
make it possible to interpolate a meter quadrat for the study of the
undergrowth at any point. The intervals of the tape are numbered from left
to right, as conspicuously and clearly as possible. For this a waterproof ink
or paint is very desirable. For holding the tapes in position, hatpins, nails,
and meat-skewers have been used with more or less satisfaction. The ideal
stake, however, is one which holds the tape close to the ground, and can be
readily moved. It is merely a stout wire, 3 mm. in diameter and 8 inches
long, looped at the top, sharpened at the tip, and with a small ring of solder
3 inches from the tip.
203. Locating quadrats. In staking a quadrat, the end tapes are
invariably placed so that the numbers read from left to right, and the side
tapes so that they read down. In mapping, a fifth tape is stretched parallel to
perquadrat one of sixteen units, or four meters square. Quadrats of greater
size are necessary in woodland and forest, where the rule, however, is that
the woody plants alone are recorded for the whole quadrat, the herbaceous
growth being listed or mapped for but one or two representative units. For
special purposes, quadrats of 3, 5, 6, etc., meters may be used, but they are
much less convenient. Quadrats are further distinguished with respect to
their use. A list quadrat is one in which the plants are merely listed and the
number of individuals of each species indicated. Chart quadrats are those in
which the area concerned is accurately mapped on plotting paper. Both list
and chart quadrats are rendered permanent by careful labeling, so that their
changes can be followed from year to year. The greater value of the chart
causes practically all permanent quadrats to be of this type, and for the
same reason only permanent chart quadrats are converted into denuded
ones.
202. Tapes and stakes. The lines for marking out quadrats are made of
strong white tape, ⅝ inches wide. This is doubled and sewed firmly at both
edges. Under moderate stretching, the tape is carefully marked off into
decimeters, and eyelets 5 mm. in diameter are set in at each end and at the
marks. This can readily be done by any shoemaker at slight expense. The
usual lengths are one and two meters, as these are most frequently used, and
they can also be easily combined to make larger quadrats. The tapes are
slightly longer than one meter in order that the distance between the end
eyelets may be exact. The tapes of the larger forest quadrats should be
divided into lengths of one meter, as these permit ready plotting and also
make it possible to interpolate a meter quadrat for the study of the
undergrowth at any point. The intervals of the tape are numbered from left
to right, as conspicuously and clearly as possible. For this a waterproof ink
or paint is very desirable. For holding the tapes in position, hatpins, nails,
and meat-skewers have been used with more or less satisfaction. The ideal
stake, however, is one which holds the tape close to the ground, and can be
readily moved. It is merely a stout wire, 3 mm. in diameter and 8 inches
long, looped at the top, sharpened at the tip, and with a small ring of solder
3 inches from the tip.
203. Locating quadrats. In staking a quadrat, the end tapes are
invariably placed so that the numbers read from left to right, and the side
tapes so that they read down. In mapping, a fifth tape is stretched parallel to
Page 219
the top, and as each decimeter strip is marked, the outer tape is shifted to
delimit the new strip. Indeed, the side tapes can be placed alone, and the
plotting tapes moved down one at a time as the mapping proceeds, but it is
usually more satisfactory to locate the quadrat exactly and to square it first,
a task most easily done by enclosing the whole quadrat, and then using a
fifth tape. In the case of list quadrats in open vegetation, the measuring strip
is unnecessary, but as a rule it facilitates counting, as well as mapping.
Fig. 50. Mapping a major quadrat on Mount Garfield at 3,600 m.
The List Quadrat
204. Description. This, as the simplest form of quadrat, is employed
primarily to ascertain the abundance of species in a formation or during a
delimit the new strip. Indeed, the side tapes can be placed alone, and the
plotting tapes moved down one at a time as the mapping proceeds, but it is
usually more satisfactory to locate the quadrat exactly and to square it first,
a task most easily done by enclosing the whole quadrat, and then using a
fifth tape. In the case of list quadrats in open vegetation, the measuring strip
is unnecessary, but as a rule it facilitates counting, as well as mapping.
Fig. 50. Mapping a major quadrat on Mount Garfield at 3,600 m.
The List Quadrat
204. Description. This, as the simplest form of quadrat, is employed
primarily to ascertain the abundance of species in a formation or during a
Page 220
particular aspect of it. Since this can be obtained readily from the chart, the
list quadrat has fallen more and more into disuse, except where it is desired
to determine abundance alone, or to aid in deciding whether a chart is really
representative. The size depends almost wholly upon the nature of the
vegetation. When the number of trees is to be determined, a quadrat of 10 or
50 meters is necessary. In ordinary herbaceous formations, the usual size is
2 meters, while the meter quadrat is used when the plants are especially
small and crowded, as in alpine meadows. The location of the quadrat is
based upon the general rule, but since its especial task is the determination
of the greatest variable in vegetation, viz., number, it is necessary to use
more quadrats, and to place them in areas which show the greatest
differences in the mixture of species. For example, it was found that a half
dozen list quadrats, when carefully located in the prairie formation, gave
results almost identical with those obtainable from a larger number. With a
little experience, the various degrees of mixture can be picked out
superficially, and the corresponding number of quadrats established. If a
single list quadrat is to be made for a formation or station, such a time
should be selected as will make it possible to cover the greatest number of
plants. Fortunately, this usually falls near the middle of the summer, when
the remains of spring plants are still in evidence, and the autumn ones are
sufficiently developed to be recognizable. In taking the census of different
aspects, the quadrat should be made as near the middle of the period as is
possible.
205. Manner of use. In listing a quadrat, i. e., counting the individuals of
each species, the plan followed is to list the smaller, less conspicuous plants
first, since they are apt to be tramped down. As a rule, the outside tapes and
the taller species afford sufficient landmarks. When this is not the case, the
measure tape is used, and the individuals of all species are checked as they
are found, while in the first method one species, rarely two, is taken at a
time. In cases of peculiar difficulty, it may be permissible to pull or break
plants as they are counted, but ordinarily this can and should be avoided.
Clusters, and bunches of stems from the same root are counted as single
plants, and the number of stems indicated by an exponent. In the case of
bunch grasses, each bunch counts as one plant.
206. Table of abundance. The species are arranged in the final list in the
order of their numerical importance, and are divided into groups which
correspond to the different degrees of abundance. The latter are arranged in
list quadrat has fallen more and more into disuse, except where it is desired
to determine abundance alone, or to aid in deciding whether a chart is really
representative. The size depends almost wholly upon the nature of the
vegetation. When the number of trees is to be determined, a quadrat of 10 or
50 meters is necessary. In ordinary herbaceous formations, the usual size is
2 meters, while the meter quadrat is used when the plants are especially
small and crowded, as in alpine meadows. The location of the quadrat is
based upon the general rule, but since its especial task is the determination
of the greatest variable in vegetation, viz., number, it is necessary to use
more quadrats, and to place them in areas which show the greatest
differences in the mixture of species. For example, it was found that a half
dozen list quadrats, when carefully located in the prairie formation, gave
results almost identical with those obtainable from a larger number. With a
little experience, the various degrees of mixture can be picked out
superficially, and the corresponding number of quadrats established. If a
single list quadrat is to be made for a formation or station, such a time
should be selected as will make it possible to cover the greatest number of
plants. Fortunately, this usually falls near the middle of the summer, when
the remains of spring plants are still in evidence, and the autumn ones are
sufficiently developed to be recognizable. In taking the census of different
aspects, the quadrat should be made as near the middle of the period as is
possible.
205. Manner of use. In listing a quadrat, i. e., counting the individuals of
each species, the plan followed is to list the smaller, less conspicuous plants
first, since they are apt to be tramped down. As a rule, the outside tapes and
the taller species afford sufficient landmarks. When this is not the case, the
measure tape is used, and the individuals of all species are checked as they
are found, while in the first method one species, rarely two, is taken at a
time. In cases of peculiar difficulty, it may be permissible to pull or break
plants as they are counted, but ordinarily this can and should be avoided.
Clusters, and bunches of stems from the same root are counted as single
plants, and the number of stems indicated by an exponent. In the case of
bunch grasses, each bunch counts as one plant.
206. Table of abundance. The species are arranged in the final list in the
order of their numerical importance, and are divided into groups which
correspond to the different degrees of abundance. The latter are arranged in
Page 221
two series, based upon the fact that association is by groups or by
individuals. The table of abundance, based upon a 2–meter quadrat rather
than upon the 5–meter one, by means of which the earlier results were
obtained, is as follows:
Social exclusive, no other species of vascular plants present
social inclusive, above 100
gr 1 gregarious 1 100–50 copious1 cop1
gr2 gregarious2 50–25 copious2 cop2
gr3 gregarious3 25–10 copious3 cop3
sg subgregarious 10–5 subcopious sc
vg vixgregarious 5–1 sparse sp
It is obvious that the above outline is faulty inasmuch as it takes no account
of the height and width of the individuals. This is a serious defect, and it
constitutes one of the many reasons why the list quadrat should be replaced
by the chart quadrat. The prairie formation affords an unusually striking
illustration of this. A single quadrat may be filled by ten plants of Psoralea
floribunda, and at the same time contain 22,000 plants of Festuca octoflora.
Yet the former is conspicuous and controlling, the latter plays an altogether
insignificant role. This difference is readily shown by comparing a plant of
each. The one is 3 × 3 feet, the other 3 × ¼ inch. Such figures furnish a
valuable check upon mere number, but make the brief, graphic designation
of abundance difficult. An attempt has been made to solve this problem by
roughly determining the space occupied by the plant, by means of the
formula, height (πR2) × abundance. This would give Psoralea a value of
210, and Festuca one of 1.6, which much more nearly represents their real
importance in the formation. Abundance or numerical value is a floristic
concept entirely, and has little place in ecology unless checked in the way
indicated. The whole problem, ecologically, depends upon an intimate
knowledge of competition, and its solution in consequence is at present
impossible.
The Chart Quadrat
207. Description and use. The detailed labor required in mapping makes
it advisable to use the meter quadrat. An additional reason of much
individuals. The table of abundance, based upon a 2–meter quadrat rather
than upon the 5–meter one, by means of which the earlier results were
obtained, is as follows:
Social exclusive, no other species of vascular plants present
social inclusive, above 100
gr 1 gregarious 1 100–50 copious1 cop1
gr2 gregarious2 50–25 copious2 cop2
gr3 gregarious3 25–10 copious3 cop3
sg subgregarious 10–5 subcopious sc
vg vixgregarious 5–1 sparse sp
It is obvious that the above outline is faulty inasmuch as it takes no account
of the height and width of the individuals. This is a serious defect, and it
constitutes one of the many reasons why the list quadrat should be replaced
by the chart quadrat. The prairie formation affords an unusually striking
illustration of this. A single quadrat may be filled by ten plants of Psoralea
floribunda, and at the same time contain 22,000 plants of Festuca octoflora.
Yet the former is conspicuous and controlling, the latter plays an altogether
insignificant role. This difference is readily shown by comparing a plant of
each. The one is 3 × 3 feet, the other 3 × ¼ inch. Such figures furnish a
valuable check upon mere number, but make the brief, graphic designation
of abundance difficult. An attempt has been made to solve this problem by
roughly determining the space occupied by the plant, by means of the
formula, height (πR2) × abundance. This would give Psoralea a value of
210, and Festuca one of 1.6, which much more nearly represents their real
importance in the formation. Abundance or numerical value is a floristic
concept entirely, and has little place in ecology unless checked in the way
indicated. The whole problem, ecologically, depends upon an intimate
knowledge of competition, and its solution in consequence is at present
impossible.
The Chart Quadrat
207. Description and use. The detailed labor required in mapping makes
it advisable to use the meter quadrat. An additional reason of much
Page 222
importance is furnished by the desirability of securing a detail photograph
of the quadrat. This is impossible with field cameras, which should not
exceed 6½ x 8½ inches, and are indeed most serviceable in the 4 × 5 size, if
the area be larger. In open formations, the major quadrat of 2 meters can be
used if necessary, but this is very rarely the case. Forest quadrats of ten
meters square are easily charted, but detail photographs can not be made of
them. Larger quadrats are impracticable; they can be counted but not
mapped to advantage. The location of the chart quadrat must be decided by
the structure to be studied. Its greatest service is in connection with zones
and societies of the same formation, which can be easily compared in the
chart form. In fact, the chart quadrat may well be regarded as the
fundamental method for inquiry into zonation and alternation. It is an
important aid in delimiting areas from the contiguous formations, and in
determining the relationships of mixed formations. It is also used to record
the character of the different aspects, but this is done more satisfactorily by
the permanent quadrat.
208. The chart used is a decimeter square, and the scale is consequently
10 : 1. It is outlined on centimeter plotting paper, and the centimeter squares
are numbered at the edges to correspond to the intervals of the quadrat, i. e.,
the top and bottom lines are numbered from left to right, and the side lines
from top to bottom. These outlines are ruled in quantity and used as needed,
or the forms can be furnished by the printer. In practice, a special quadrat
book the size of the chart has been used. The need of a second book may be
avoided by outlining two charts on the plotting sheet, and filing the latter in
the field record book. In the few cases where 2–meter quadrats are
desirable, four charts are used, care being taken to label them so that they
can be combined whenever necessary. Ten-meter quadrats are recorded on
the decimeter chart also, each meter interval corresponding to a centimeter,
i. e., the scale is 100 : 1.
of the quadrat. This is impossible with field cameras, which should not
exceed 6½ x 8½ inches, and are indeed most serviceable in the 4 × 5 size, if
the area be larger. In open formations, the major quadrat of 2 meters can be
used if necessary, but this is very rarely the case. Forest quadrats of ten
meters square are easily charted, but detail photographs can not be made of
them. Larger quadrats are impracticable; they can be counted but not
mapped to advantage. The location of the chart quadrat must be decided by
the structure to be studied. Its greatest service is in connection with zones
and societies of the same formation, which can be easily compared in the
chart form. In fact, the chart quadrat may well be regarded as the
fundamental method for inquiry into zonation and alternation. It is an
important aid in delimiting areas from the contiguous formations, and in
determining the relationships of mixed formations. It is also used to record
the character of the different aspects, but this is done more satisfactorily by
the permanent quadrat.
208. The chart used is a decimeter square, and the scale is consequently
10 : 1. It is outlined on centimeter plotting paper, and the centimeter squares
are numbered at the edges to correspond to the intervals of the quadrat, i. e.,
the top and bottom lines are numbered from left to right, and the side lines
from top to bottom. These outlines are ruled in quantity and used as needed,
or the forms can be furnished by the printer. In practice, a special quadrat
book the size of the chart has been used. The need of a second book may be
avoided by outlining two charts on the plotting sheet, and filing the latter in
the field record book. In the few cases where 2–meter quadrats are
desirable, four charts are used, care being taken to label them so that they
can be combined whenever necessary. Ten-meter quadrats are recorded on
the decimeter chart also, each meter interval corresponding to a centimeter,
i. e., the scale is 100 : 1.
Page 223
Fig. 51. Permanent chart quadrat, Andosacile, Carex-Campanulacoryphium.
Page 224
Fig. 52. Chart of the quadrat shown in figure 51. Legend: a, Androsace chamaejasme; c, Carex
rupestris; t, Tetraneuris lanata; p, Potentilla rubricaulis; as, Arenaria sajanensis; ar, Artemisia
scopulorum; ag, Agropyrum scribneri; sa, Silene acaulis; st, Sieversia turbinata; d, Dasyphora
fruticosa; al, Allium reticulatum; o, Oreoxis alpina.
209. Mapping is invariably begun at the upper left-hand corner of the
chart, and is carried across the strip marked off by the plotting tape,
decimeter by decimeter. As soon as this strip is completed, a second one is
formed by moving the top tape to a position one decimeter below the
plotting tape, which then becomes the upper one. This is repeated until the
last strip is reached. Little difficulty is experienced in locating each plant
rupestris; t, Tetraneuris lanata; p, Potentilla rubricaulis; as, Arenaria sajanensis; ar, Artemisia
scopulorum; ag, Agropyrum scribneri; sa, Silene acaulis; st, Sieversia turbinata; d, Dasyphora
fruticosa; al, Allium reticulatum; o, Oreoxis alpina.
209. Mapping is invariably begun at the upper left-hand corner of the
chart, and is carried across the strip marked off by the plotting tape,
decimeter by decimeter. As soon as this strip is completed, a second one is
formed by moving the top tape to a position one decimeter below the
plotting tape, which then becomes the upper one. This is repeated until the
last strip is reached. Little difficulty is experienced in locating each plant
Page 225
exactly, as the decimeter interval is small, and the centimeter square which
corresponds is divided into twenty-five tiny squares. Each plant is put in
whenever possible, but mats, turfs, and mosses are merely outlined in mass
if the individuals are not distinguishable. This holds true of all large rosettes
and mats, even when they are single plants. Symbols were formerly used
for indicating the various species. They have the advantage of requiring
little space on the chart, and the disadvantage of necessitating constant
reference to the legend. They are at present replaced by initials. By this
plan, the decapitalized first letter of the generic name is used if no other
genus found in the quadrat begins with the same letter. If, however, two or
more genera begin with a, for example Agropyrum, Anemone, and Allium,
the most abundant one is indicated by a, and the others by the first two
letters, as an, al. In case two species of the same genus are present, the
species initial is used in connection with that for the genus, as ac and ar for
Agropyrum caninum and Agropyrum richardsonii respectively. It is rarely
necessary to exceed two letters for any species. Plants which regularly have
several stems from the same root are indicated by the initial and an
exponent as a3. Seedlings are represented by a line drawn through the letter.
Usually the chart sheet affords sufficient space below the chart for the
legend. When the list of species is long, the back of the sheet is used.
210. Factors and photographs. Each chart is numbered, and the
formation, station, and date indicated. The constant factors, altitude, slope,
and exposure are ascertained and recorded on the sheet. The variable factors
are read in each quadrat whenever possible, and in addition to being
preserved in the record book, are noted on the chart sheet along with the
base reading in the formation for the same time. This facilitates the
interpretation of the differences found when two or more charts are
compared. Chart quadrats are regularly photographed. For this purpose a
long focus 4 × 5 camera with a telephoto lens is used. At the proper distance
this will make a view of the same size as the chart, thus making possible an
exact comparison of the two. The chart and photograph serve as mutual
checks, as well as complements, since the former shows number, position,
and arrangement, and the latter, height, form, position, and arrangement.
The view is usually made by placing the camera directly in front of the
middle of the lower tape, at such a distance that the side tapes fall just
within the limits of the ground glass. The swing is always used in order that
the focus may be uniformly sharp. Surface views of the quadrat can be
corresponds is divided into twenty-five tiny squares. Each plant is put in
whenever possible, but mats, turfs, and mosses are merely outlined in mass
if the individuals are not distinguishable. This holds true of all large rosettes
and mats, even when they are single plants. Symbols were formerly used
for indicating the various species. They have the advantage of requiring
little space on the chart, and the disadvantage of necessitating constant
reference to the legend. They are at present replaced by initials. By this
plan, the decapitalized first letter of the generic name is used if no other
genus found in the quadrat begins with the same letter. If, however, two or
more genera begin with a, for example Agropyrum, Anemone, and Allium,
the most abundant one is indicated by a, and the others by the first two
letters, as an, al. In case two species of the same genus are present, the
species initial is used in connection with that for the genus, as ac and ar for
Agropyrum caninum and Agropyrum richardsonii respectively. It is rarely
necessary to exceed two letters for any species. Plants which regularly have
several stems from the same root are indicated by the initial and an
exponent as a3. Seedlings are represented by a line drawn through the letter.
Usually the chart sheet affords sufficient space below the chart for the
legend. When the list of species is long, the back of the sheet is used.
210. Factors and photographs. Each chart is numbered, and the
formation, station, and date indicated. The constant factors, altitude, slope,
and exposure are ascertained and recorded on the sheet. The variable factors
are read in each quadrat whenever possible, and in addition to being
preserved in the record book, are noted on the chart sheet along with the
base reading in the formation for the same time. This facilitates the
interpretation of the differences found when two or more charts are
compared. Chart quadrats are regularly photographed. For this purpose a
long focus 4 × 5 camera with a telephoto lens is used. At the proper distance
this will make a view of the same size as the chart, thus making possible an
exact comparison of the two. The chart and photograph serve as mutual
checks, as well as complements, since the former shows number, position,
and arrangement, and the latter, height, form, position, and arrangement.
The view is usually made by placing the camera directly in front of the
middle of the lower tape, at such a distance that the side tapes fall just
within the limits of the ground glass. The swing is always used in order that
the focus may be uniformly sharp. Surface views of the quadrat can be
Page 226
taken by means of a device which permits the camera to hang downward
from the tripod, or by means of a tripod with a swinging platform. Such
views are especially valuable for the study of competition, since they give a
clear idea of the spread and density of the various plants. They are difficult
to make unless the vegetation is low and nearly uniform in height. The
usual photograph is much more serviceable in regular quadrat work.
The Permanent Quadrat
211. Description and uses. As stated heretofore, either list or chart
quadrats may be rendered permanent in order that they may be followed
from season to season or from year to year. As a matter of fact, however, an
area which is to be studied repeatedly really demands charting, and in
practice chart quadrats alone are made permanent. This is done simply by
driving a labeled stake at one corner of the quadrat, and locating the latter
definitely in relation to a conspicuous landmark. When one is in residence
for several years, practically all chart quadrats are converted into permanent
ones, since the work already done in the chart quadrat is so much
accomplished towards the permanent one. This is not necessary when one
wishes merely to compare different areas of stable formations. As a rule,
however, some change is constantly being wrought by invasion or
competition, and the amount and direction of this can only be revealed by
the permanent quadrat. The latter has a fundamental value for all kinds of
invasion, but it is absolutely indispensable in studying complete invasion or
succession, and in discovering and recording the gradual effects of
competition. It is in the detailed investigation of these dynamic phenomena
that the paramount importance of the quadrat is most evident. If the
experience of several years be taken as conclusive, no other method is
capable of revealing the minute changes as they are occurring.
from the tripod, or by means of a tripod with a swinging platform. Such
views are especially valuable for the study of competition, since they give a
clear idea of the spread and density of the various plants. They are difficult
to make unless the vegetation is low and nearly uniform in height. The
usual photograph is much more serviceable in regular quadrat work.
The Permanent Quadrat
211. Description and uses. As stated heretofore, either list or chart
quadrats may be rendered permanent in order that they may be followed
from season to season or from year to year. As a matter of fact, however, an
area which is to be studied repeatedly really demands charting, and in
practice chart quadrats alone are made permanent. This is done simply by
driving a labeled stake at one corner of the quadrat, and locating the latter
definitely in relation to a conspicuous landmark. When one is in residence
for several years, practically all chart quadrats are converted into permanent
ones, since the work already done in the chart quadrat is so much
accomplished towards the permanent one. This is not necessary when one
wishes merely to compare different areas of stable formations. As a rule,
however, some change is constantly being wrought by invasion or
competition, and the amount and direction of this can only be revealed by
the permanent quadrat. The latter has a fundamental value for all kinds of
invasion, but it is absolutely indispensable in studying complete invasion or
succession, and in discovering and recording the gradual effects of
competition. It is in the detailed investigation of these dynamic phenomena
that the paramount importance of the quadrat is most evident. If the
experience of several years be taken as conclusive, no other method is
capable of revealing the minute changes as they are occurring.
Page 227
Fig. 53. Permanent quadrat, Polygonile (Polygonum bistortoides) Ruxton Park; mapped and
photographed July 22, denuded September 8, 1903.
The permanent quadrat is regularly 1 meter square, a size determined
both by the exigencies of charting and photographing. When ecograph
batteries are used, the quadrat is located as close to the latter as is possible.
Otherwise, the quadrat itself should constitute a station for making factor
observations. This connection is absolutely essential, since the quadrat is
used expressly to determine the structural changes, which are produced by
physical factors, and the reaction of vegetation upon them. Permanent
quadrats are established in different formations or stages of a succession to
trace the invasion of new species and the dropping out of old ones in
response to competition. They serve to distinguish the proper formation,
which represents a particular stage of development, from the mixed
formations which precede and follow, and also to determine the exact
photographed July 22, denuded September 8, 1903.
The permanent quadrat is regularly 1 meter square, a size determined
both by the exigencies of charting and photographing. When ecograph
batteries are used, the quadrat is located as close to the latter as is possible.
Otherwise, the quadrat itself should constitute a station for making factor
observations. This connection is absolutely essential, since the quadrat is
used expressly to determine the structural changes, which are produced by
physical factors, and the reaction of vegetation upon them. Permanent
quadrats are established in different formations or stages of a succession to
trace the invasion of new species and the dropping out of old ones in
response to competition. They serve to distinguish the proper formation,
which represents a particular stage of development, from the mixed
formations which precede and follow, and also to determine the exact
Page 228
course as well as the rapidity of the change that follows each reaction.
When applied to different examples of the same stage, and to all the
different stages of a succession, the whole development of the latter may be
minutely traced and definitely recorded. The importance of following the
changes from aspect to aspect is much less, since these are periodical rather
than dynamic. They are an essential feature of structure, however, and it has
been the practice to make at least one series of aspect charts from each
permanent quadrat.
For tracing the invasion and competition of lichens and mosses, which
play a primary role in initial formations, a subquadrat is used. The size
varies, but it is usually smaller than the quadrat, although the latter is
entirely available in the case of the large foliose lichens. For the crustose
and smaller foliose forms, a subquadrat 2 decimeters square is used, and for
the larger forms and tufted mosses, one of 5 decimeters. In the case of
ground forms, tapes are employed, and the quadrat is permanently staked.
On rocks and cliffs, where moss and lichen stages are most common, tapes
are impracticable, and the quadrat is permanently outlined with paint.
Charts of lichen quadrats are made to the usual scale of 10 : 1.
212. Manner of use. Permanent quadrats are mapped and photographed
in exactly the same way as chart quadrats. As soon as this has been done, a
labeled stake is driven at the upper left-hand corner, so that its edge
indicates the exact position of the quadrat stake, and a smaller one is placed
at the opposite corner to facilitate the task of setting the tapes accurately in
later readings. The label stake bears merely the number of the quadrat and
the date when it was first established. It is firmly fixed and allowed to
project just enough to enable it to be located readily. Its position requires
careful landmarking when the quadrat is to be visited year by year. In forest
formations, this is readily done by blazing, but in grassland it is necessary
to have recourse to compass and pacing, or to erect an artificial landmark.
After several charts have been made, a permanent quadrat attains a high
value, and every precaution must be taken to prevent losing its exact
location. At the second reading of a quadrat, whether in the succeeding
aspect or year, the tapes are placed with reference to the stakes, and a chart
and photograph are made in the usual manner. These are labeled and dated
like the original ones, but they are numbered to indicate both the quadrat
and the series, e. g., 152 indicates the second chart, and photograph made of
quadrat 15. The date indicates whether the readings are by the aspect or the
When applied to different examples of the same stage, and to all the
different stages of a succession, the whole development of the latter may be
minutely traced and definitely recorded. The importance of following the
changes from aspect to aspect is much less, since these are periodical rather
than dynamic. They are an essential feature of structure, however, and it has
been the practice to make at least one series of aspect charts from each
permanent quadrat.
For tracing the invasion and competition of lichens and mosses, which
play a primary role in initial formations, a subquadrat is used. The size
varies, but it is usually smaller than the quadrat, although the latter is
entirely available in the case of the large foliose lichens. For the crustose
and smaller foliose forms, a subquadrat 2 decimeters square is used, and for
the larger forms and tufted mosses, one of 5 decimeters. In the case of
ground forms, tapes are employed, and the quadrat is permanently staked.
On rocks and cliffs, where moss and lichen stages are most common, tapes
are impracticable, and the quadrat is permanently outlined with paint.
Charts of lichen quadrats are made to the usual scale of 10 : 1.
212. Manner of use. Permanent quadrats are mapped and photographed
in exactly the same way as chart quadrats. As soon as this has been done, a
labeled stake is driven at the upper left-hand corner, so that its edge
indicates the exact position of the quadrat stake, and a smaller one is placed
at the opposite corner to facilitate the task of setting the tapes accurately in
later readings. The label stake bears merely the number of the quadrat and
the date when it was first established. It is firmly fixed and allowed to
project just enough to enable it to be located readily. Its position requires
careful landmarking when the quadrat is to be visited year by year. In forest
formations, this is readily done by blazing, but in grassland it is necessary
to have recourse to compass and pacing, or to erect an artificial landmark.
After several charts have been made, a permanent quadrat attains a high
value, and every precaution must be taken to prevent losing its exact
location. At the second reading of a quadrat, whether in the succeeding
aspect or year, the tapes are placed with reference to the stakes, and a chart
and photograph are made in the usual manner. These are labeled and dated
like the original ones, but they are numbered to indicate both the quadrat
and the series, e. g., 152 indicates the second chart, and photograph made of
quadrat 15. The date indicates whether the readings are by the aspect or the
Page 229
year, though this may be shown also in the name of the series itself. It is
clearly an advantage to have the two successive charts of a quadrat upon the
same sheet, and to file all the charts and photographs of the same permanent
quadrat together, and in the proper order.
Since much of the value of a permanent quadrat depends upon its use as a
station for observing physical factors, it is unprofitable to establish a large
number. The results of invasion and competition can be ascertained by the
quadrat alone, but these should be merely preliminary to seeking for their
causes. Clearly, a quadrat should be established for each battery of
instruments, while additional ones should be located only in so far as they
can be visited often enough to give an insight into the factors that control
them. In view of the fact that the most important factors, water-content and
light, are less variable than humidity, temperature, and wind, it will suffice
if visits are made once a week. This is especially true when it is possible to
refer the more variable factors to the continuous records of a base station.
While all the results determined for permanent quadrats are preserved in the
field record, a record of them is also kept on the reverse of the chart sheet
for convenience in interpreting the different charts.
The Denuded Quadrat
213. Description. This is primarily a permanent quadrat from which the
plant covering has been removed, after it has been charted and
photographed. What is practically the same thing is obtained by establishing
a permanent quadrat in a new soil, or in one recently laid bare and not yet
reclothed with plants. These, however, are merely permanent quadrats, in
which the first chart and photograph furnish a record of the habitat alone.
They are of great importance in succession, and will be more fully
discussed under experimental vegetation. The denuded quadrat is of the
usual size, 1 meter, though the smaller lichen quadrats are also denuded.
The location is subject to the conditions already indicated, especially with
reference to physical factors. The denuded quadrat, however, is particularly
adapted to the study of invasion and the resulting competition.
Consequently, when migration is markedly from one direction, a series of
denuded quadrats throws a flood of light upon the actual steps in invasion.
Denuding is a valuable aid in succession, but it must be clearly recognized
clearly an advantage to have the two successive charts of a quadrat upon the
same sheet, and to file all the charts and photographs of the same permanent
quadrat together, and in the proper order.
Since much of the value of a permanent quadrat depends upon its use as a
station for observing physical factors, it is unprofitable to establish a large
number. The results of invasion and competition can be ascertained by the
quadrat alone, but these should be merely preliminary to seeking for their
causes. Clearly, a quadrat should be established for each battery of
instruments, while additional ones should be located only in so far as they
can be visited often enough to give an insight into the factors that control
them. In view of the fact that the most important factors, water-content and
light, are less variable than humidity, temperature, and wind, it will suffice
if visits are made once a week. This is especially true when it is possible to
refer the more variable factors to the continuous records of a base station.
While all the results determined for permanent quadrats are preserved in the
field record, a record of them is also kept on the reverse of the chart sheet
for convenience in interpreting the different charts.
The Denuded Quadrat
213. Description. This is primarily a permanent quadrat from which the
plant covering has been removed, after it has been charted and
photographed. What is practically the same thing is obtained by establishing
a permanent quadrat in a new soil, or in one recently laid bare and not yet
reclothed with plants. These, however, are merely permanent quadrats, in
which the first chart and photograph furnish a record of the habitat alone.
They are of great importance in succession, and will be more fully
discussed under experimental vegetation. The denuded quadrat is of the
usual size, 1 meter, though the smaller lichen quadrats are also denuded.
The location is subject to the conditions already indicated, especially with
reference to physical factors. The denuded quadrat, however, is particularly
adapted to the study of invasion and the resulting competition.
Consequently, when migration is markedly from one direction, a series of
denuded quadrats throws a flood of light upon the actual steps in invasion.
Denuding is a valuable aid in succession, but it must be clearly recognized
Page 230
that, while permanent quadrats register the exact course of the succession,
denuded ones can merely furnish facts as to the probable courses of stages
not now in evidence.
Fig. 54. Denuded quadrat; this is the quadrat shown in figure 53; photographed September 7, 1904.
214. Methods of denuding and recording. Permanent quadrats may be
denuded at any time during the time they are under observation. The best
results, however, are to be obtained by establishing the two side by side, or
at least close together. In this way, they are mutually supplementary, and
furnish the most evidence possible with regard to the procedure of invasion
and competition. Another advantage is found in that the same observations
of climatic factors will do for both, though water-content and soil
temperatures are necessarily different. A quadrat which is to be denuded is
first mapped, photographed, and labeled exactly like a permanent quadrat.
denuded ones can merely furnish facts as to the probable courses of stages
not now in evidence.
Fig. 54. Denuded quadrat; this is the quadrat shown in figure 53; photographed September 7, 1904.
214. Methods of denuding and recording. Permanent quadrats may be
denuded at any time during the time they are under observation. The best
results, however, are to be obtained by establishing the two side by side, or
at least close together. In this way, they are mutually supplementary, and
furnish the most evidence possible with regard to the procedure of invasion
and competition. Another advantage is found in that the same observations
of climatic factors will do for both, though water-content and soil
temperatures are necessarily different. A quadrat which is to be denuded is
first mapped, photographed, and labeled exactly like a permanent quadrat.
Page 231
The vegetation is then destroyed. This is usually done by removal, though it
may also be burnt, destroyed by flooding, or in some other manner. The
method will depend upon the use which the quadrat is to serve. If it is to
throw light upon the vegetation of an area in which denudation has affected
the surface alone, the aerial parts only are removed by paring the surface
with a spade. When the disturbance is to be more profound, the upper seed-
bearing layer is removed, and the underground parts dug up. In the
interpretation of a secondary succession, the denuding cause is made use of
in a fashion as nearly natural as possible. Ordinarily, the plants are removed
just below the top of the ground by a spade, leaving the underground parts
undisturbed. This method has yielded very interesting results.
Quadrats have been denuded in the fall after the majority of the plants
have completed their growth. This is largely owing to the fact that other
field work is less pressing at this time. Denudation can be done as well in
the spring, though the invasion will be slower in this case, since the seeds
which have accumulated will be partly or entirely removed. During the first
season the denuded quadrat should be mapped every month, and, if the
invasion be rapid, photographed also. In open formations, especially those
of a xerophytic nature, a single chart and photograph made at the end of the
season are sufficient. In a few cases of this sort, indeed, no invaders have
appeared until the second year. Beginning with the second season, a single
record taken near the close of the growing period will suffice. Denuded
quadrats are labeled, dated, and filed exactly as other permanent quadrats,
but it should be noted that the first member of the chart and photograph
series is that which records the original vegetation of the area denuded.
215. Physical factors. When denuded quadrats are single, their physical
factors must be observed in the usual way. If they are associated with
permanent ones, the ordinary readings are made for the latter, and those
factors which are affected by exposing the soil are alone taken for the
denuded area. These are the water-content, soil and surface temperatures,
and in some stations at least the humidity near the surface. As everywhere,
water-content is the most important, but the temperature at or near the
surface has a marked effect upon germination. Because of its bearing upon
the latter, the surface water-content is usually determined also. This has
been done by taking a surface sample 2 inches square and 1 inch deep.
Denuded quadrats naturally show considerable differences from year to
may also be burnt, destroyed by flooding, or in some other manner. The
method will depend upon the use which the quadrat is to serve. If it is to
throw light upon the vegetation of an area in which denudation has affected
the surface alone, the aerial parts only are removed by paring the surface
with a spade. When the disturbance is to be more profound, the upper seed-
bearing layer is removed, and the underground parts dug up. In the
interpretation of a secondary succession, the denuding cause is made use of
in a fashion as nearly natural as possible. Ordinarily, the plants are removed
just below the top of the ground by a spade, leaving the underground parts
undisturbed. This method has yielded very interesting results.
Quadrats have been denuded in the fall after the majority of the plants
have completed their growth. This is largely owing to the fact that other
field work is less pressing at this time. Denudation can be done as well in
the spring, though the invasion will be slower in this case, since the seeds
which have accumulated will be partly or entirely removed. During the first
season the denuded quadrat should be mapped every month, and, if the
invasion be rapid, photographed also. In open formations, especially those
of a xerophytic nature, a single chart and photograph made at the end of the
season are sufficient. In a few cases of this sort, indeed, no invaders have
appeared until the second year. Beginning with the second season, a single
record taken near the close of the growing period will suffice. Denuded
quadrats are labeled, dated, and filed exactly as other permanent quadrats,
but it should be noted that the first member of the chart and photograph
series is that which records the original vegetation of the area denuded.
215. Physical factors. When denuded quadrats are single, their physical
factors must be observed in the usual way. If they are associated with
permanent ones, the ordinary readings are made for the latter, and those
factors which are affected by exposing the soil are alone taken for the
denuded area. These are the water-content, soil and surface temperatures,
and in some stations at least the humidity near the surface. As everywhere,
water-content is the most important, but the temperature at or near the
surface has a marked effect upon germination. Because of its bearing upon
the latter, the surface water-content is usually determined also. This has
been done by taking a surface sample 2 inches square and 1 inch deep.
Denuded quadrats naturally show considerable differences from year to
Page 232
year as the action of the invaders becomes more pronounced. To this fact is
due much of their value as aids in interpreting succession.
Aquatic Quadrats
216. Scope. The preceding discussion of quadrat methods is based
wholly upon their use in terrestrial formations. Wet meadow and dry bog
are the wettest places in which quadrats have been used. It is clear,
however, that with certain necessary modifications, quadrats can be used as
successfully, though not as conveniently, in many water formations as in
land ones. The tapes need to be raised above the surface of the water by
longer stakes, and photographs often taken from a boat, but otherwise the
usual methods apply, at any rate for bogs and shallow bodies of water. In
lakes or streams the tapes might be attached to buoys or floats. The
determination of factors is made as usual. Permanent quadrats are feasible
in many cases at least, and denuded quadrats are not altogether impossible.
due much of their value as aids in interpreting succession.
Aquatic Quadrats
216. Scope. The preceding discussion of quadrat methods is based
wholly upon their use in terrestrial formations. Wet meadow and dry bog
are the wettest places in which quadrats have been used. It is clear,
however, that with certain necessary modifications, quadrats can be used as
successfully, though not as conveniently, in many water formations as in
land ones. The tapes need to be raised above the surface of the water by
longer stakes, and photographs often taken from a boat, but otherwise the
usual methods apply, at any rate for bogs and shallow bodies of water. In
lakes or streams the tapes might be attached to buoys or floats. The
determination of factors is made as usual. Permanent quadrats are feasible
in many cases at least, and denuded quadrats are not altogether impossible.
Page 233
TRANSECTS
217. The transect is essentially a cross section through the vegetation of
a station, a formation, or a series of formations. It is designed primarily to
show the order of arrangement of species in zones and societies, but it also
serves as a record of the heterogeneity of any area. In the form of the layer
transect, it furnishes a graphic method of representing the spatial relations
of the species in layered formations, e. g., forests, ponds, and lakes. It is
merely a logical extension of the idea underlying the quadrat, and the
transect is, indeed, little more than an elongated quadrat. An important
difference, however, lies in the fact that the former normally traverses areas
more or less unlike, while the latter is always located in a homogeneous
one. Furthermore, the transect is plotted with especial reference to the
topography. With respect to dimension, transects are classified as line, layer,
and belt transects, and the latter may also be permanent or denuded.
The Line Transect
218. Description and method. A simple transect is sometimes made by
establishing the points between which it is to be run, and then recording the
plants pace by pace along this line. This is satisfactory where the striking
changes in structure are desired. A more accurate method is ordinarily used,
since it gives detailed results, and at the same time brings out the more
general features. For this, use is made of a tape of proper length which is
divided into decimeters. Tapes of 10, 50, and 100 meters are used, and if
they are furnished with eyelets, transects of intermediate lengths may be run
with them. When longer transects are desired, as in the case of forest
formations, tapes of 500 or 1,000 meters should be used with eyelets a
meter apart. The transect is located in the area to be studied by running the
tape from one landmark to another, fastening it here and there by means of
quadrat stakes. Previous to this, the shortest distance between landmarks is
ascertained when the transect runs through a depression or upon a level
surface. In the case of an elevation, the height is ascertained by a barometer,
the length and angle of the two slopes obtained, and the length of the base
217. The transect is essentially a cross section through the vegetation of
a station, a formation, or a series of formations. It is designed primarily to
show the order of arrangement of species in zones and societies, but it also
serves as a record of the heterogeneity of any area. In the form of the layer
transect, it furnishes a graphic method of representing the spatial relations
of the species in layered formations, e. g., forests, ponds, and lakes. It is
merely a logical extension of the idea underlying the quadrat, and the
transect is, indeed, little more than an elongated quadrat. An important
difference, however, lies in the fact that the former normally traverses areas
more or less unlike, while the latter is always located in a homogeneous
one. Furthermore, the transect is plotted with especial reference to the
topography. With respect to dimension, transects are classified as line, layer,
and belt transects, and the latter may also be permanent or denuded.
The Line Transect
218. Description and method. A simple transect is sometimes made by
establishing the points between which it is to be run, and then recording the
plants pace by pace along this line. This is satisfactory where the striking
changes in structure are desired. A more accurate method is ordinarily used,
since it gives detailed results, and at the same time brings out the more
general features. For this, use is made of a tape of proper length which is
divided into decimeters. Tapes of 10, 50, and 100 meters are used, and if
they are furnished with eyelets, transects of intermediate lengths may be run
with them. When longer transects are desired, as in the case of forest
formations, tapes of 500 or 1,000 meters should be used with eyelets a
meter apart. The transect is located in the area to be studied by running the
tape from one landmark to another, fastening it here and there by means of
quadrat stakes. Previous to this, the shortest distance between landmarks is
ascertained when the transect runs through a depression or upon a level
surface. In the case of an elevation, the height is ascertained by a barometer,
the length and angle of the two slopes obtained, and the length of the base
Page 234
line determined from these data. The field record of the arrangement of the
plants is made entirely without reference to the surface line. The vertical
lines on the centimeter sheet are taken to correspond with the tape, and the
individual which touches the latter on either side is recorded to the right or
left respectively and within the proper square. The species are indicated as
for quadrats. A single row on either side may be taken alone, but the double
series serves as a desirable check. After the record is made, the topography
of the transect is drawn carefully to scale. This drawing is made upon the
scale of 100 : 1 for transects of 10 meters or less, and of 1000 : 1 for those
that are longer. The combination of this drawing with the line series of
plants can not be made advantageously in the field. For the shorter
transects, meter sizes of centimeter plotting paper can often be used to
advantage. In this event, the topographic line is drawn to the scale of 10 : 1
and the series of plants transferred directly to it. In the case of transects
between 10 and 100 meters, the scale of the drawing is increased from 1000
: 1 to 100 : 1, so that each decimeter of the original series is compressed
into a centimeter. For the longest transects, corresponding reductions must
be made, but in these it will be remembered that the series is plotted by
meter instead of decimeter.
219. The location and size of line transects are determined by the
purpose for which they are designed. Short transects are valuable for detail,
but they can be used to advantage only where changes in arrangement are
taking place rapidly. They are especially adapted to the study of minute
alternations and to the zonation of small ponds, streams, ditches, roads,
blowouts, etc. Longer transects can not furnish the same detail, on account
of the amount of time necessary, but they are invaluable for the zonation
and alternation of larger areas, such as the consocies, formation, and
formation series. They are of particular importance for the record of
zonation, since they afford a clue to the topographic symmetry of the area.
The location of a transect depends upon the area to be studied, though it
should always run through a portion as typical as possible. The general
direction is ascertained by means of the compass, and when there is a
measurable difference in elevation it is taken by the barometer or otherwise.
The points at which ecotones cross the transect are carefully indicated
upon the chart. They serve as stations for simultaneous readings of physical
factors, though in the majority of cases water-content readings alone will
determine the reason for the ecotone. Photographs of line transects should
plants is made entirely without reference to the surface line. The vertical
lines on the centimeter sheet are taken to correspond with the tape, and the
individual which touches the latter on either side is recorded to the right or
left respectively and within the proper square. The species are indicated as
for quadrats. A single row on either side may be taken alone, but the double
series serves as a desirable check. After the record is made, the topography
of the transect is drawn carefully to scale. This drawing is made upon the
scale of 100 : 1 for transects of 10 meters or less, and of 1000 : 1 for those
that are longer. The combination of this drawing with the line series of
plants can not be made advantageously in the field. For the shorter
transects, meter sizes of centimeter plotting paper can often be used to
advantage. In this event, the topographic line is drawn to the scale of 10 : 1
and the series of plants transferred directly to it. In the case of transects
between 10 and 100 meters, the scale of the drawing is increased from 1000
: 1 to 100 : 1, so that each decimeter of the original series is compressed
into a centimeter. For the longest transects, corresponding reductions must
be made, but in these it will be remembered that the series is plotted by
meter instead of decimeter.
219. The location and size of line transects are determined by the
purpose for which they are designed. Short transects are valuable for detail,
but they can be used to advantage only where changes in arrangement are
taking place rapidly. They are especially adapted to the study of minute
alternations and to the zonation of small ponds, streams, ditches, roads,
blowouts, etc. Longer transects can not furnish the same detail, on account
of the amount of time necessary, but they are invaluable for the zonation
and alternation of larger areas, such as the consocies, formation, and
formation series. They are of particular importance for the record of
zonation, since they afford a clue to the topographic symmetry of the area.
The location of a transect depends upon the area to be studied, though it
should always run through a portion as typical as possible. The general
direction is ascertained by means of the compass, and when there is a
measurable difference in elevation it is taken by the barometer or otherwise.
The points at which ecotones cross the transect are carefully indicated
upon the chart. They serve as stations for simultaneous readings of physical
factors, though in the majority of cases water-content readings alone will
determine the reason for the ecotone. Photographs of line transects should
Page 235
be made while the tape is in
position, in order that the
superficies of the series may be as
evident as possible.
The Belt Transect
220. Details. This differs from
the line transect in that it is wider,
and consequently affords a more
accurate record of the arrangement
of plants. While both give the
actual facts of distribution, the line
transect necessarily ignores the
minor lateral deviations in position.
These are brought out in a strip of
some width, and the belt transect
thus gives a more correct view of
the variations which result from
competition in an area physically
homogeneous. The width of such
transects depends upon the length,
and the character of the vegetation.
The standard width is one
decimeter in herbaceous
formations, and one meter in the
long transects which are used in
woodlands. In open vegetation,
especially in the initial stages of
successions, the width may often
be increased to advantage, but
ordinarily the amount of work
necessary to run a belt transect of
some length limits the width to one
decimeter.
position, in order that the
superficies of the series may be as
evident as possible.
The Belt Transect
220. Details. This differs from
the line transect in that it is wider,
and consequently affords a more
accurate record of the arrangement
of plants. While both give the
actual facts of distribution, the line
transect necessarily ignores the
minor lateral deviations in position.
These are brought out in a strip of
some width, and the belt transect
thus gives a more correct view of
the variations which result from
competition in an area physically
homogeneous. The width of such
transects depends upon the length,
and the character of the vegetation.
The standard width is one
decimeter in herbaceous
formations, and one meter in the
long transects which are used in
woodlands. In open vegetation,
especially in the initial stages of
successions, the width may often
be increased to advantage, but
ordinarily the amount of work
necessary to run a belt transect of
some length limits the width to one
decimeter.
Page 236
The location of a belt transect,
the choice of landmarks, the
determination of direction and
elevation are made exactly as for
the line transect. The topographic
map is made in precisely the same
way also, the scale used depending
upon the length. Two tapes,
however, are employed, and these
are placed so that they mark off a
strip just one decimeter wide.
Every few meters, or oftener if
need be, they are checked by a
decimeter rule, and fixed firmly in
place by quadrat stakes. The
arrangement of the plants is
recorded as for the line transect,
except that the record covers a
decimeter strip just as in quadrat
work. Accordingly, an interval of a
centimeter is left on the sheet
between the successive portions of
the strip, in order that the latter
may be put together without
confusion when the topographic
map and the plant series are
combined. The record should
invariably start in the upper left-
hand corner and read down. The
map and the centimeter strip
recording the plants of the transect
are combined on a common scale
as already indicated for the line
transect.
The ecotones of zones are shown
on belt transects by single cross
lines, and those of consocies by
the choice of landmarks, the
determination of direction and
elevation are made exactly as for
the line transect. The topographic
map is made in precisely the same
way also, the scale used depending
upon the length. Two tapes,
however, are employed, and these
are placed so that they mark off a
strip just one decimeter wide.
Every few meters, or oftener if
need be, they are checked by a
decimeter rule, and fixed firmly in
place by quadrat stakes. The
arrangement of the plants is
recorded as for the line transect,
except that the record covers a
decimeter strip just as in quadrat
work. Accordingly, an interval of a
centimeter is left on the sheet
between the successive portions of
the strip, in order that the latter
may be put together without
confusion when the topographic
map and the plant series are
combined. The record should
invariably start in the upper left-
hand corner and read down. The
map and the centimeter strip
recording the plants of the transect
are combined on a common scale
as already indicated for the line
transect.
The ecotones of zones are shown
on belt transects by single cross
lines, and those of consocies by
Page 237
Fig. 55. Line transect running east
parallel cross lines. In taking
and west in the Picea-Pinus-hylium,
photographs of the transect, it is
showing the relation of the desirable to use guidons to mark
herbaceous layer to the Carex-Catha-
these points clearly. The same
helium, invading along the brook;
device may also be used to indicate
ecotones at e. the course of the transect, when the
tapes are completely hidden by the
plants. Physical factor readings
should always be taken, and, as before, they are best made at the
intersections of the ecotones.
The Permanent Transect
221. Advantages. Both line and belt transects, after they have been
recorded, should be rendered permanent, in order that they may serve to
indicate the changes of a heterogeneous area from year to year in the same
detailed fashion that the permanent quadrat does for homogeneous ones.
For historical as well as for physical reasons, the ecotones of zones and of
consocies are subject to change from year to year, and the amount and
direction of this change can only be ascertained from annual records made
in exactly the same spot. By means of the permanent transect alone the very
origin of such areas can be followed from one stage to another of the
succession. Moreover, the transect is equally valuable with the quadrat in
making it possible to follow every step of the minute changes wrought by
competition.
222. Details. The transect is made permanent by blazing the landmarks at
either end, if these already exist, or by erecting them when it is necessary. A
label stake is driven at each end, on which is painted the number and date of
the transect and its length. Each stake should also indicate the exact
direction in which the other lies. The position of the ecotone is indicated by
smaller stakes bearing the number of the transect and the date when the
ecotone was found at that point. These are left in place, and in a few years
show very graphically the change in position of the zones. For the first
season, permanent transects afford results of great value when recorded for
each aspect, but after this an annual visit will suffice. The details of
mapping, plotting, etc., are identical with those indicated above, with the
parallel cross lines. In taking
and west in the Picea-Pinus-hylium,
photographs of the transect, it is
showing the relation of the desirable to use guidons to mark
herbaceous layer to the Carex-Catha-
these points clearly. The same
helium, invading along the brook;
device may also be used to indicate
ecotones at e. the course of the transect, when the
tapes are completely hidden by the
plants. Physical factor readings
should always be taken, and, as before, they are best made at the
intersections of the ecotones.
The Permanent Transect
221. Advantages. Both line and belt transects, after they have been
recorded, should be rendered permanent, in order that they may serve to
indicate the changes of a heterogeneous area from year to year in the same
detailed fashion that the permanent quadrat does for homogeneous ones.
For historical as well as for physical reasons, the ecotones of zones and of
consocies are subject to change from year to year, and the amount and
direction of this change can only be ascertained from annual records made
in exactly the same spot. By means of the permanent transect alone the very
origin of such areas can be followed from one stage to another of the
succession. Moreover, the transect is equally valuable with the quadrat in
making it possible to follow every step of the minute changes wrought by
competition.
222. Details. The transect is made permanent by blazing the landmarks at
either end, if these already exist, or by erecting them when it is necessary. A
label stake is driven at each end, on which is painted the number and date of
the transect and its length. Each stake should also indicate the exact
direction in which the other lies. The position of the ecotone is indicated by
smaller stakes bearing the number of the transect and the date when the
ecotone was found at that point. These are left in place, and in a few years
show very graphically the change in position of the zones. For the first
season, permanent transects afford results of great value when recorded for
each aspect, but after this an annual visit will suffice. The details of
mapping, plotting, etc., are identical with those indicated above, with the
Page 238
addition that all charts and photographs must bear the number of the
reading as well as that of the transect. Physical factor observations are taken
as often as the charts are made, and the results noted on the back of the
chart sheet for purposes of ready comparison.
The Denuded Transect
223. The denuded transect bears exactly the same relation to a permanent
one as that which exists between the denuded and the permanent quadrat.
While the permanent transect records the actual mutations due to changing
physical factors or to competition, the denuded transect throws needed light
upon the mobility and ecesis of the various species, and upon the nature of
the competition between them. Denuded transects may be established
wherever it seems desirable, after the strip has been properly charted and
photographed. The most valuable results, however, are secured by locating
each one alongside of a permanent one. The best plan is to locate and chart
two permanent transects a meter apart. A single view is then made of the
two. One of them is denuded together with a strip 2 decimeters on either
side, resulting in a denuded transect 5 decimeters wide. In charting this
during succeeding years, the entire width may well be plotted as long as the
vegetation is open, but after it has again become well established, it is
necessary to save time by confining one’s attention to the central decimeter
strip. Photographs can be made either of the permanent and denuded
transects singly, or of the two together. The latter method has certain
obvious advantages. Climatic factor readings can be made for both transects
in common, but all those factors which are affected by the exposure of the
soil surface must be observed in each.
The Layer Transect
224. This is a modification of the line transect, by means of which the
vertical relations of plants are also shown, especially the tendency to form
layers which is so regular a feature of forest formations. Owing to the
difficulty of charting in three planes, belt transects do not lend themselves
to this purpose. Because of the greater complexity, layer transects can rarely
reading as well as that of the transect. Physical factor observations are taken
as often as the charts are made, and the results noted on the back of the
chart sheet for purposes of ready comparison.
The Denuded Transect
223. The denuded transect bears exactly the same relation to a permanent
one as that which exists between the denuded and the permanent quadrat.
While the permanent transect records the actual mutations due to changing
physical factors or to competition, the denuded transect throws needed light
upon the mobility and ecesis of the various species, and upon the nature of
the competition between them. Denuded transects may be established
wherever it seems desirable, after the strip has been properly charted and
photographed. The most valuable results, however, are secured by locating
each one alongside of a permanent one. The best plan is to locate and chart
two permanent transects a meter apart. A single view is then made of the
two. One of them is denuded together with a strip 2 decimeters on either
side, resulting in a denuded transect 5 decimeters wide. In charting this
during succeeding years, the entire width may well be plotted as long as the
vegetation is open, but after it has again become well established, it is
necessary to save time by confining one’s attention to the central decimeter
strip. Photographs can be made either of the permanent and denuded
transects singly, or of the two together. The latter method has certain
obvious advantages. Climatic factor readings can be made for both transects
in common, but all those factors which are affected by the exposure of the
soil surface must be observed in each.
The Layer Transect
224. This is a modification of the line transect, by means of which the
vertical relations of plants are also shown, especially the tendency to form
layers which is so regular a feature of forest formations. Owing to the
difficulty of charting in three planes, belt transects do not lend themselves
to this purpose. Because of the greater complexity, layer transects can rarely
Page 239
exceed ten meters in length except in those formations where layering is
little or not at all developed. The simplest method is to establish a line
transect in the ordinary way, and then to record the height of each plant as
its position is noted. This is done by means of a measuring stick ruled in
decimeters, which can be moved from interval to interval along the tape, or
better, by two such sticks connected by tapes a meter long at every five
decimeters of the sticks. These should be two meters high for woodland,
and one meter for grassland. Layer transects often run on even surfaces, but
if this is not the case, the usual data for a topographic map should be taken.
The final chart is constructed on the scale of 10:1, the height of each plant
being indicated by a vertical line equal to .1 of the observed height. A
photograph of a representative meter of the transect is taken when the
measuring sticks and rods indicated above are in position. Physical factor
readings, principally of light, but often also of humidity, temperature, and
wind are made at the height of the various layers when these are present.
little or not at all developed. The simplest method is to establish a line
transect in the ordinary way, and then to record the height of each plant as
its position is noted. This is done by means of a measuring stick ruled in
decimeters, which can be moved from interval to interval along the tape, or
better, by two such sticks connected by tapes a meter long at every five
decimeters of the sticks. These should be two meters high for woodland,
and one meter for grassland. Layer transects often run on even surfaces, but
if this is not the case, the usual data for a topographic map should be taken.
The final chart is constructed on the scale of 10:1, the height of each plant
being indicated by a vertical line equal to .1 of the observed height. A
photograph of a representative meter of the transect is taken when the
measuring sticks and rods indicated above are in position. Physical factor
readings, principally of light, but often also of humidity, temperature, and
wind are made at the height of the various layers when these are present.
Page 240
ECOTONE CHARTS
225. The contour lines of zones and consocies are of the utmost
importance in recording the structure of vegetation. They do not permit
such accuracy as do quadrats and transects, but this is hardly to be
considered a disadvantage in view of the fact that ecotones are rarely
sharply defined. In establishing the ecotones of zonation, the width and the
length of the base, i. e., the area of excess or deficiency, or as much of it as
is to be considered, are determined. This base may be road, ditch, pool,
lake, or stream, or the peak or crest of a hill, ridge, or mountain. When the
zonation is bilateral, meter tapes are run at right angles to the base, at
proper intervals, and the points and the distances where the ecotones cross
are noted. In the case of radial zones, the tapes are run in the four cardinal
directions, and if the base be large, in the four intermediate ones also, the
intersections being likewise noted. From the data thus obtained, the zones
may be outlined with a fair degree of accuracy. If the series be an extensive
one, it is charted to the scale of 100:1; in cases of small areas, however, the
scale of 10:1 will give better results. Whenever the zones show clearly
enough to warrant, a photograph is also taken. Water-content readings are of
paramount importance in the interpretation of zones. Samples should be
taken at all intersections, and the resulting values indicated at the
corresponding points upon the chart. When the zones are broken up into
alternating patches in consequence of asymmetry in the topography, the
ecotones of the latter are traced in a similar fashion from the center of each
as a base, the absolute position of which is ultimately determined with
reference to the ecotone lines already established.
225. The contour lines of zones and consocies are of the utmost
importance in recording the structure of vegetation. They do not permit
such accuracy as do quadrats and transects, but this is hardly to be
considered a disadvantage in view of the fact that ecotones are rarely
sharply defined. In establishing the ecotones of zonation, the width and the
length of the base, i. e., the area of excess or deficiency, or as much of it as
is to be considered, are determined. This base may be road, ditch, pool,
lake, or stream, or the peak or crest of a hill, ridge, or mountain. When the
zonation is bilateral, meter tapes are run at right angles to the base, at
proper intervals, and the points and the distances where the ecotones cross
are noted. In the case of radial zones, the tapes are run in the four cardinal
directions, and if the base be large, in the four intermediate ones also, the
intersections being likewise noted. From the data thus obtained, the zones
may be outlined with a fair degree of accuracy. If the series be an extensive
one, it is charted to the scale of 100:1; in cases of small areas, however, the
scale of 10:1 will give better results. Whenever the zones show clearly
enough to warrant, a photograph is also taken. Water-content readings are of
paramount importance in the interpretation of zones. Samples should be
taken at all intersections, and the resulting values indicated at the
corresponding points upon the chart. When the zones are broken up into
alternating patches in consequence of asymmetry in the topography, the
ecotones of the latter are traced in a similar fashion from the center of each
as a base, the absolute position of which is ultimately determined with
reference to the ecotone lines already established.
Page 241
THE MIGRATION CIRCLE
226. Purpose. The migration circle is designed to record the invasion of
species, since it operates outward from an individual or a group of plants as
a center. As migration takes place to a certain degree in all directions, a
circle is better adapted to the purpose than the quadrat. From the very
nature of invasion, migration circles should always he permanent in order
that the yearly advance may be accurately noted. Circles of this character
are important aids in the study of any vegetation, except, perhaps, one that
has practically become stabilized. Their great value, however, is found in
succession, where it is necessary to trace the movement of new individuals
away from the original invaders as centers of colonization.
227. Location and method. The size of the migration circle is largely
controlled by the density of the vegetation, and in some degree by the
height of the species also, since this determines the trajectory of the
disseminule. In close formations, a circle of 1–, rarely of 5–meter radius can
best be used, but in the more open initial stages of succession a radius of 5,
10, or, in exceptional cases such as open woodland, even 25 meters, affords
the best results. The location should always be made with a plant or group
of plants of the species to be studied as a center. This migration circle
differs from the quadrat in that it is used to show the movement of one,
rarely two or three species, and not the position of all the plants within it.
The center is permanently fixed by driving a labeled stake with the number
of the circle and the data. Two tapes the length of the radius are used for
recording. These are provided with the usual eyelets, 5 decimeters apart,
and are fastened on a peg in the top of the central stake so that they move
readily. At the outer ends they are staked 5 decimeters apart by a tape of this
length when the radius is 1 meter, and 1 meter when the radius is 5 or 10
meters. The record forms must be especially prepared on blank sheets about
9 inches square. The scale is 10:1 for circles of 1 meter, and 100:1 for those
of 5– and 10–meter radius. In the former, concentric circles are drawn about
the center at intervals of 5 decimeters, and radii are drawn to the
circumference at the same interval. In the larger circles, the intervals are 1
meter. Each segment of the circle is read by means of the two tapes, and the
position indicated with reference to the concentric lines and radii. When but
226. Purpose. The migration circle is designed to record the invasion of
species, since it operates outward from an individual or a group of plants as
a center. As migration takes place to a certain degree in all directions, a
circle is better adapted to the purpose than the quadrat. From the very
nature of invasion, migration circles should always he permanent in order
that the yearly advance may be accurately noted. Circles of this character
are important aids in the study of any vegetation, except, perhaps, one that
has practically become stabilized. Their great value, however, is found in
succession, where it is necessary to trace the movement of new individuals
away from the original invaders as centers of colonization.
227. Location and method. The size of the migration circle is largely
controlled by the density of the vegetation, and in some degree by the
height of the species also, since this determines the trajectory of the
disseminule. In close formations, a circle of 1–, rarely of 5–meter radius can
best be used, but in the more open initial stages of succession a radius of 5,
10, or, in exceptional cases such as open woodland, even 25 meters, affords
the best results. The location should always be made with a plant or group
of plants of the species to be studied as a center. This migration circle
differs from the quadrat in that it is used to show the movement of one,
rarely two or three species, and not the position of all the plants within it.
The center is permanently fixed by driving a labeled stake with the number
of the circle and the data. Two tapes the length of the radius are used for
recording. These are provided with the usual eyelets, 5 decimeters apart,
and are fastened on a peg in the top of the central stake so that they move
readily. At the outer ends they are staked 5 decimeters apart by a tape of this
length when the radius is 1 meter, and 1 meter when the radius is 5 or 10
meters. The record forms must be especially prepared on blank sheets about
9 inches square. The scale is 10:1 for circles of 1 meter, and 100:1 for those
of 5– and 10–meter radius. In the former, concentric circles are drawn about
the center at intervals of 5 decimeters, and radii are drawn to the
circumference at the same interval. In the larger circles, the intervals are 1
meter. Each segment of the circle is read by means of the two tapes, and the
position indicated with reference to the concentric lines and radii. When but
Page 242
one species is read, a tiny circle is used to denote the position of each plant.
If more than one is used, the symbols are those already indicated for the
quadrat. One tape is left in place and the other with the segment tape is
shifted to a new position, and the resulting segment is read as before. The
exact position of the base radius is fixed by a label stake, in order that the
segments of successive years may exactly correspond. The record sheet is
labeled, dated, and filed. By folding at one edge, it may be filed in the
regular field book.
228. The denuded circle is established in the same way as a permanent
one. The original position of the individuals of the species under
consideration may be recorded or not, depending upon the use to be made
of the results. The safest plan is first to read the circle in the usual way, and
then to denude it. The latter should be done in such a way as to remove all
the disseminules from the surface in so far as possible. It is essential also
that this be done before the seeds are mature and begin to be scattered. The
central plant or cluster is of course not removed. In special cases, all the
plants of the species are allowed to remain to serve as centers of
colonization. The successive yearly readings of the denuded area are made
exactly as for a permanent circle. Permanent and denuded circles, like
quadrats, should always be established near each other so that they permit
of ready comparison under similar conditions.
229. Photographs of migration circles furnish the most detail when the
camera is placed just behind the central group in such a way as to show its
relation to the other individuals or clusters of the circle. In the denuded
circle, or when the plants stand out conspicuously from the bulk of the
vegetation, it is not necessary to use guidons, but in other cases the latter
greatly increase the value of the picture. Factor readings are less important
for migration circles than for quadrats and transects. The factors of principal
importance are those that deal with migration and ecesis, i. e., wind, water-
content, and soil temperatures. The former may be determined for both
circles in common, but the conditions that affect ecesis must be observed
separately for each.
If more than one is used, the symbols are those already indicated for the
quadrat. One tape is left in place and the other with the segment tape is
shifted to a new position, and the resulting segment is read as before. The
exact position of the base radius is fixed by a label stake, in order that the
segments of successive years may exactly correspond. The record sheet is
labeled, dated, and filed. By folding at one edge, it may be filed in the
regular field book.
228. The denuded circle is established in the same way as a permanent
one. The original position of the individuals of the species under
consideration may be recorded or not, depending upon the use to be made
of the results. The safest plan is first to read the circle in the usual way, and
then to denude it. The latter should be done in such a way as to remove all
the disseminules from the surface in so far as possible. It is essential also
that this be done before the seeds are mature and begin to be scattered. The
central plant or cluster is of course not removed. In special cases, all the
plants of the species are allowed to remain to serve as centers of
colonization. The successive yearly readings of the denuded area are made
exactly as for a permanent circle. Permanent and denuded circles, like
quadrats, should always be established near each other so that they permit
of ready comparison under similar conditions.
229. Photographs of migration circles furnish the most detail when the
camera is placed just behind the central group in such a way as to show its
relation to the other individuals or clusters of the circle. In the denuded
circle, or when the plants stand out conspicuously from the bulk of the
vegetation, it is not necessary to use guidons, but in other cases the latter
greatly increase the value of the picture. Factor readings are less important
for migration circles than for quadrats and transects. The factors of principal
importance are those that deal with migration and ecesis, i. e., wind, water-
content, and soil temperatures. The former may be determined for both
circles in common, but the conditions that affect ecesis must be observed
separately for each.
Page 243
CARTOGRAPHY
230. Value of cartographic methods. Chart, map, and photograph are
records indispensable to the systematic study of vegetation. They serve not
merely to preserve the facts ascertained, and to permit their ready
comparison, but they also put a premium upon accurate methods, and
consequently bring to light many points otherwise overlooked. For ecology,
they have the value which drawings possess in taxonomy, in that they make
clear at a glance what pages of description fail to indicate. They are the
fundamental material of comparative phytogeography, and in all careful
vegetational study their use is no longer optional but obligatory. Hence it is
obvious that cartographic methods should be clear and simple, and that they
should be uniform, so that charts and maps of widely separated formations
may be directly compared without difficulty. It is not to be expected that
uniform methods will come into general use immediately, but a proper
appreciation of the obligation that rests upon every ecologist to make his
results both easily comprehensible and usable will serve to produce this
very necessary result. In the treatment that follows, as elsewhere, no
attempt is made to describe the general cartographic methods used by other
ecologists, notably Flahault. The methods employed by the author form a
complete system, which has proved valuable, and for various reasons it
alone is discussed here.
231. Standard scale. The question of the scale to which charts and maps
are to be made is of primary importance. The general principle is that the
ratio between area and drawing should be as small as possible. Moreover,
charts and maps of the same character should always be drawn to the same
scale, unless a good reason to the contrary exists. The ideal scale is 1:1,
which is manifestly an impossibility. This is approached most nearly in the
quadrat chart where the scale is 10:1. Charts of definite areas are made on a
scale as large as possible, while maps of formations, regions, etc., are
necessarily drawn upon a very small scale. General maps designed to show
the distribution of species and formations, or the vegetation of continents,
are usually not drawn with reference to a scale at all. While it is manifestly
impossible to use the same scale for charts and maps, it is feasible and
desirable that they be constructed upon scales readily convertible into each
230. Value of cartographic methods. Chart, map, and photograph are
records indispensable to the systematic study of vegetation. They serve not
merely to preserve the facts ascertained, and to permit their ready
comparison, but they also put a premium upon accurate methods, and
consequently bring to light many points otherwise overlooked. For ecology,
they have the value which drawings possess in taxonomy, in that they make
clear at a glance what pages of description fail to indicate. They are the
fundamental material of comparative phytogeography, and in all careful
vegetational study their use is no longer optional but obligatory. Hence it is
obvious that cartographic methods should be clear and simple, and that they
should be uniform, so that charts and maps of widely separated formations
may be directly compared without difficulty. It is not to be expected that
uniform methods will come into general use immediately, but a proper
appreciation of the obligation that rests upon every ecologist to make his
results both easily comprehensible and usable will serve to produce this
very necessary result. In the treatment that follows, as elsewhere, no
attempt is made to describe the general cartographic methods used by other
ecologists, notably Flahault. The methods employed by the author form a
complete system, which has proved valuable, and for various reasons it
alone is discussed here.
231. Standard scale. The question of the scale to which charts and maps
are to be made is of primary importance. The general principle is that the
ratio between area and drawing should be as small as possible. Moreover,
charts and maps of the same character should always be drawn to the same
scale, unless a good reason to the contrary exists. The ideal scale is 1:1,
which is manifestly an impossibility. This is approached most nearly in the
quadrat chart where the scale is 10:1. Charts of definite areas are made on a
scale as large as possible, while maps of formations, regions, etc., are
necessarily drawn upon a very small scale. General maps designed to show
the distribution of species and formations, or the vegetation of continents,
are usually not drawn with reference to a scale at all. While it is manifestly
impossible to use the same scale for charts and maps, it is feasible and
desirable that they be constructed upon scales readily convertible into each
Page 244
other. This is most satisfactorily accomplished by means of the decimal
system, and the various type scales are 10:1, 100:1, 1000:1, etc. The first
two or three scales are used for charts of quadrats, transects, and circles; the
remaining ones are employed in making maps of large areas. No attempt
has been made to draw an absolute line between charts and maps, but an
endeavor is made to restrict the term chart to the record of the number and
position of plants, while maps deal with the arrangement and location of
formational areas. It is hardly necessary to point out the reasons why all
charts and maps should be based upon the decimal system of scales.
Experience will furnish the very best of arguments.
232. Color scheme. The first requisite for the graphic representation of
formations, regions, etc., is that each class of formations be invariably
indicated by the same color. It is also necessary that the colors and shades
be easily distinguishable, and it is at least desirable that they be referred to
the different classes in some consistent sequence. Uniformity in all these
points is greatly to be desired at the hands of all ecologists. Here, as in the
case of the standard scale, uniformity will be found the more desirable the
more impossible it is made by ignoring it. In the use of color to represent
regions and provinces, on maps too small to indicate formations, the color
of each division is represented by the color of its dominant formation; thus
the prairie province is colored ochroleucus on account of the color used to
represent prairie formations, the boreal-subalpine zone atrovirens on
account of the typical coniferous forests, etc. No endeavor has been made to
take account of the various types of formations, e. g., the different
coniferous forests, as this is a problem to be worked out for more local
maps in various shades of dark green, etc. The following color scheme
which has been based upon the points made above is proposed as a
satisfactory solution of the problem. The color standard used is that of
Saccardo’s Chromotaxia.
I. Hydrophytic Formations: blue
1. Marine: cyaneus
2. Brackish: ardesiacus
3. Freshwater: caeruleus
4. Swamps and marshes: caesius
system, and the various type scales are 10:1, 100:1, 1000:1, etc. The first
two or three scales are used for charts of quadrats, transects, and circles; the
remaining ones are employed in making maps of large areas. No attempt
has been made to draw an absolute line between charts and maps, but an
endeavor is made to restrict the term chart to the record of the number and
position of plants, while maps deal with the arrangement and location of
formational areas. It is hardly necessary to point out the reasons why all
charts and maps should be based upon the decimal system of scales.
Experience will furnish the very best of arguments.
232. Color scheme. The first requisite for the graphic representation of
formations, regions, etc., is that each class of formations be invariably
indicated by the same color. It is also necessary that the colors and shades
be easily distinguishable, and it is at least desirable that they be referred to
the different classes in some consistent sequence. Uniformity in all these
points is greatly to be desired at the hands of all ecologists. Here, as in the
case of the standard scale, uniformity will be found the more desirable the
more impossible it is made by ignoring it. In the use of color to represent
regions and provinces, on maps too small to indicate formations, the color
of each division is represented by the color of its dominant formation; thus
the prairie province is colored ochroleucus on account of the color used to
represent prairie formations, the boreal-subalpine zone atrovirens on
account of the typical coniferous forests, etc. No endeavor has been made to
take account of the various types of formations, e. g., the different
coniferous forests, as this is a problem to be worked out for more local
maps in various shades of dark green, etc. The following color scheme
which has been based upon the points made above is proposed as a
satisfactory solution of the problem. The color standard used is that of
Saccardo’s Chromotaxia.
I. Hydrophytic Formations: blue
1. Marine: cyaneus
2. Brackish: ardesiacus
3. Freshwater: caeruleus
4. Swamps and marshes: caesius
Page 245
II. Mesophytic Formations
A. Forest formations: green
1. Coniferous forests: atrovirens
2. Broadleaved evergreen forests: viridis
3. Deciduous forests: flavovirens
B. Grassland formations: yellow
1. Meadows: melleus
2. Prairies: ochroleucus
C. Culture and waste formations: red
1. Fields: ruber
2. Groves and orchards: atropurpureus
3. Wastes: purpureus
III. Xerophytic Formations: brown
1. Deserts: isabellinus
2. Plains and steppes: avellaneus
3. Saline formations: umbrinus
4. Arctic-alpine formations: testaceus
233. Formation and vegetation maps are detailed maps of a single
formation or a series of them, showing the formational limits, and when the
scale is not too small, the ecotones of zones and consocies. In the cases
where the topography is level, as sometimes happens in mapping single
formations, the chain and pedometer must be used to ascertain the size of
the different areas. Indeed in all mapping of vegetation, the methods of
surveying are directly applicable. Over large areas, however, it is not
necessary that limits be drawn with mathematical accuracy, and for the
purposes of the ecologist, the plane table and camera are satisfactory
A. Forest formations: green
1. Coniferous forests: atrovirens
2. Broadleaved evergreen forests: viridis
3. Deciduous forests: flavovirens
B. Grassland formations: yellow
1. Meadows: melleus
2. Prairies: ochroleucus
C. Culture and waste formations: red
1. Fields: ruber
2. Groves and orchards: atropurpureus
3. Wastes: purpureus
III. Xerophytic Formations: brown
1. Deserts: isabellinus
2. Plains and steppes: avellaneus
3. Saline formations: umbrinus
4. Arctic-alpine formations: testaceus
233. Formation and vegetation maps are detailed maps of a single
formation or a series of them, showing the formational limits, and when the
scale is not too small, the ecotones of zones and consocies. In the cases
where the topography is level, as sometimes happens in mapping single
formations, the chain and pedometer must be used to ascertain the size of
the different areas. Indeed in all mapping of vegetation, the methods of
surveying are directly applicable. Over large areas, however, it is not
necessary that limits be drawn with mathematical accuracy, and for the
purposes of the ecologist, the plane table and camera are satisfactory
Page 246
substitutes for the surveyor’s transit, at least in the present aspect of the
subject. When the formation or group of formations is commanded by an
elevation of some height, the latter is used as a base. A plane table is
established upon it and the topographical and vegetational features are
recorded in the usual way. This map is usually supplemented by a series of
views from the same base. Indeed it has come to be recognized that a
complete series of photographs of this kind give a more valuable record
than the plane table, and that the construction of an accurate map from them
is an easy matter. Since the camera saves much time and energy also, it is
used almost exclusively to furnish the data for map making. In hilly, and
especially in mountainous regions, the photographic method is
indispensable. Its application is extremely simple. A central hill or mountain
is selected, and from it a series of views is taken so that the edge of one
exactly meets the edge of the other. This is an extremely important matter,
and demands much nicety of judgment. The camera is kept in the same
spot, and after each exposure it is turned as the operator looks through it
until a landmark at one edge just passes from view at the other. As soon as
the new position is determined, the tripod screw is turned to hold the box
firmly in position. In case of a slight jar, the exact position should again be
obtained. If the series is accurately made, the resulting prints will give a
complete panoramic view of the region, without overlap or omission. For
this purpose, a 6½ × 8½ camera is desirable, since the topographic and
vegetational features are larger and stand out more distinctly. A large
camera requires fewer changes of position, and hence saves time and
reduces the chance of error. A 4 × 5 camera serves the purpose sufficiently
well, though it requires a little more care in operation on account of the
greater number of exposures necessary. This may be avoided in some
degree by the use of a wide-angle lens if the depth of the area is not too
great. Whatever camera may be used, a telephoto lens is a very desirable
adjunct, since it enables one to choose between three different sizes of the
view without changing the position of the camera. To avoid possible
confusion, the exposures are always made from right to left, and the plates
are used in the numerical order of their holders. For the same reason the
landmarks are described and numbered in their proper order. The prints
obtained are mounted on a card in sequence. The view map may be
preserved in this form, or it may be reduced or enlarged by making a copy
to the size desired. Outline maps of topography may be traced from the
subject. When the formation or group of formations is commanded by an
elevation of some height, the latter is used as a base. A plane table is
established upon it and the topographical and vegetational features are
recorded in the usual way. This map is usually supplemented by a series of
views from the same base. Indeed it has come to be recognized that a
complete series of photographs of this kind give a more valuable record
than the plane table, and that the construction of an accurate map from them
is an easy matter. Since the camera saves much time and energy also, it is
used almost exclusively to furnish the data for map making. In hilly, and
especially in mountainous regions, the photographic method is
indispensable. Its application is extremely simple. A central hill or mountain
is selected, and from it a series of views is taken so that the edge of one
exactly meets the edge of the other. This is an extremely important matter,
and demands much nicety of judgment. The camera is kept in the same
spot, and after each exposure it is turned as the operator looks through it
until a landmark at one edge just passes from view at the other. As soon as
the new position is determined, the tripod screw is turned to hold the box
firmly in position. In case of a slight jar, the exact position should again be
obtained. If the series is accurately made, the resulting prints will give a
complete panoramic view of the region, without overlap or omission. For
this purpose, a 6½ × 8½ camera is desirable, since the topographic and
vegetational features are larger and stand out more distinctly. A large
camera requires fewer changes of position, and hence saves time and
reduces the chance of error. A 4 × 5 camera serves the purpose sufficiently
well, though it requires a little more care in operation on account of the
greater number of exposures necessary. This may be avoided in some
degree by the use of a wide-angle lens if the depth of the area is not too
great. Whatever camera may be used, a telephoto lens is a very desirable
adjunct, since it enables one to choose between three different sizes of the
view without changing the position of the camera. To avoid possible
confusion, the exposures are always made from right to left, and the plates
are used in the numerical order of their holders. For the same reason the
landmarks are described and numbered in their proper order. The prints
obtained are mounted on a card in sequence. The view map may be
preserved in this form, or it may be reduced or enlarged by making a copy
to the size desired. Outline maps of topography may be traced from the
Page 247
resulting negative, and the formations filled in by means of the proper
colors. The most satisfactory method, however, is to have the original views
or the copy printed “light” and to color the formations just as they appear
there, with all the wealth of topographic and vegetational detail. If a
detailed topographic map alone is desired, this is traced directly from the
large copy.
234. Continental maps. A method of determining the general outlines of
regions, provinces, and vegetational zones as a preliminary to their detailed
study has been used successfully for several years.[22] This is based upon
provincial and continental maps on which are traced the geographical areas
of the species of genera typical of the various formations. Detail
topographic maps of the prairie province and the North American continent
have been used for this purpose. A number of the facies of extensive and
representative formations of the different portions of the continent are
selected and grouped according to genera. One map is devoted to each
genus, unless the number of species is large. In this case a number of maps
are used, since the limits are apt to become confused. The range of each
species is determined from all the reliable sources, and a corresponding line
is drawn upon the map to delimit its geographical area. The limits of the
area of each species are drawn in a different color, and the name of the
species printed in the same color in the legend. Although this work has as
yet been done only for the trees of North America, and for the grasses and
principal species of the prairie province, it promises to constitute a final
method for the limitation of vegetational divisions. It is clear that if the
original data concerning ranges are accurate, the increasing study of
formations will do little more than rectify the detailed course of the limiting
line, since in most cases facies and formations coincide in distribution. The
limiting line or ecotone of a zone or province is a composite obtained from
the limits of certain representative facies and principal species, and checked
by the limits of species typical of the contiguous vegetations. Thus, the
boreal-subalpine zone is clearly outlined by combining the limits of
Populus tremuloides, Larix americana, Pinus banksiana, Abies balsamea,
Picea mariana, Picea canadensis, and Betula papyracea, and checking the
results by the areal limits of the hardwoods and grasses to the southward.
colors. The most satisfactory method, however, is to have the original views
or the copy printed “light” and to color the formations just as they appear
there, with all the wealth of topographic and vegetational detail. If a
detailed topographic map alone is desired, this is traced directly from the
large copy.
234. Continental maps. A method of determining the general outlines of
regions, provinces, and vegetational zones as a preliminary to their detailed
study has been used successfully for several years.[22] This is based upon
provincial and continental maps on which are traced the geographical areas
of the species of genera typical of the various formations. Detail
topographic maps of the prairie province and the North American continent
have been used for this purpose. A number of the facies of extensive and
representative formations of the different portions of the continent are
selected and grouped according to genera. One map is devoted to each
genus, unless the number of species is large. In this case a number of maps
are used, since the limits are apt to become confused. The range of each
species is determined from all the reliable sources, and a corresponding line
is drawn upon the map to delimit its geographical area. The limits of the
area of each species are drawn in a different color, and the name of the
species printed in the same color in the legend. Although this work has as
yet been done only for the trees of North America, and for the grasses and
principal species of the prairie province, it promises to constitute a final
method for the limitation of vegetational divisions. It is clear that if the
original data concerning ranges are accurate, the increasing study of
formations will do little more than rectify the detailed course of the limiting
line, since in most cases facies and formations coincide in distribution. The
limiting line or ecotone of a zone or province is a composite obtained from
the limits of certain representative facies and principal species, and checked
by the limits of species typical of the contiguous vegetations. Thus, the
boreal-subalpine zone is clearly outlined by combining the limits of
Populus tremuloides, Larix americana, Pinus banksiana, Abies balsamea,
Picea mariana, Picea canadensis, and Betula papyracea, and checking the
results by the areal limits of the hardwoods and grasses to the southward.
Page 248
PHOTOGRAPHY
235. The camera is an indispensable instrument for the ecologist.
Although it has too often been employed to give an air of thoroughness to
work of no ecological value, it is as important for recording the structure of
vegetation as the automatic instrument is for the study of the habitat. No
ecologist is equipped for systematic field investigation until he is provided
with a good camera and has become skilful in its use. For this reason, it is
felt that a few hints concerning photographic methods and their application
in ecology may not be out of place. No written advice can take the place of
experience, but certain elementary suggestions and cautions will greatly
shorten the apprenticeship of one who does not have the good fortune to be
taught by a professional photographer. To the student of ecology, the camera
is not a toy. It must be understood and operated with as much thoroughness
as any other instrument, and when this is done, the results will be equally
certain and desirable.
Fig. 56. 4 × 5 long focus “Korona” camera (series V).
235. The camera is an indispensable instrument for the ecologist.
Although it has too often been employed to give an air of thoroughness to
work of no ecological value, it is as important for recording the structure of
vegetation as the automatic instrument is for the study of the habitat. No
ecologist is equipped for systematic field investigation until he is provided
with a good camera and has become skilful in its use. For this reason, it is
felt that a few hints concerning photographic methods and their application
in ecology may not be out of place. No written advice can take the place of
experience, but certain elementary suggestions and cautions will greatly
shorten the apprenticeship of one who does not have the good fortune to be
taught by a professional photographer. To the student of ecology, the camera
is not a toy. It must be understood and operated with as much thoroughness
as any other instrument, and when this is done, the results will be equally
certain and desirable.
Fig. 56. 4 × 5 long focus “Korona” camera (series V).
Page 249
Fig. 57. 5 × 7 long focus “Premo” camera.
236. The camera and its accessories. Although two cameras are
desirable whenever it is possible to obtain them, a single one will meet all
the requirements of field work. This should be 4 × 5 inches in size, since it
is much more convenient and will do all the work that a larger camera can.
In the comparatively few cases in which larger views are needed, the 4 × 5
negatives can be readily enlarged. The smaller instrument is less expensive
in operation because of the cheapness of the plates, and it gives a negative
of the proper size for lantern slides and for reproduction. A 6½ × 8½ camera
is valuable in special cases, such as making a series of photographs for
maps. In the writer’s own experience, the 6½ × 8½ camera, although used
exclusively at first, has been almost completely supplanted by the 4 × 5.
The best field camera is of the folding type with a good stout box. It must
be what is known technically as a long focus instrument, which enables
small objects to be taken natural size and permits the use of a telephoto
lens. It should be provided with a swing and also a reversible back by which
the position of the plates can be changed instantly. The lens must be of the
telephoto pattern, which makes it possible to use the front or back lens
either alone or in combination. The chief advantage of this is that the image,
when distant, may be made of three different sizes without changing the
position of the camera. Generally speaking, the high-priced rapid lenses are
the best, since it is exceptional to get the desired length of exposure in
vegetation, on account of the ease with which the plants move in the wind.
Before buying such a lens it is desirable to test its rapidity and depth of
focus, since it is not necessarily better than some of the lenses furnished
236. The camera and its accessories. Although two cameras are
desirable whenever it is possible to obtain them, a single one will meet all
the requirements of field work. This should be 4 × 5 inches in size, since it
is much more convenient and will do all the work that a larger camera can.
In the comparatively few cases in which larger views are needed, the 4 × 5
negatives can be readily enlarged. The smaller instrument is less expensive
in operation because of the cheapness of the plates, and it gives a negative
of the proper size for lantern slides and for reproduction. A 6½ × 8½ camera
is valuable in special cases, such as making a series of photographs for
maps. In the writer’s own experience, the 6½ × 8½ camera, although used
exclusively at first, has been almost completely supplanted by the 4 × 5.
The best field camera is of the folding type with a good stout box. It must
be what is known technically as a long focus instrument, which enables
small objects to be taken natural size and permits the use of a telephoto
lens. It should be provided with a swing and also a reversible back by which
the position of the plates can be changed instantly. The lens must be of the
telephoto pattern, which makes it possible to use the front or back lens
either alone or in combination. The chief advantage of this is that the image,
when distant, may be made of three different sizes without changing the
position of the camera. Generally speaking, the high-priced rapid lenses are
the best, since it is exceptional to get the desired length of exposure in
vegetation, on account of the ease with which the plants move in the wind.
Before buying such a lens it is desirable to test its rapidity and depth of
focus, since it is not necessarily better than some of the lenses furnished
Page 250
with good cameras. The lens should be provided with an iris diaphragm
capable of being stopped down to 128 or 256. The shutters furnished with
the ordinary lenses are satisfactory, since “snap-shots,” i. e., instantaneous
exposures, are practically never possible for plants. The automatic shutter of
the “Premo” camera is an especially convenient form. All shutters should be
carefully tested before using to determine the exact time value of the
exposures indicated. It is not uncommon for the exposure at 1 second, or at
other points, to have a value quite different from the one indicated. When
this is the case, it is evident that it can not be known too soon. The camera
should have at least a half-dozen double plate-holders. These are numbered
consecutively so that the figure uppermost when the holder is in the camera
will indicate the number of the plate exposed. A carrying case is desirable
on a long trip when all the plate-holders must be taken, but ordinarily it is a
disadvantage, since the camera box will carry two or three holders. The
camera cloth should be as small and light as possible, and at the same time
opaque. The most satisfactory one for the field is the rubber cloth. The
tripod should be a happy combination of lightness and stability, a condition
more nearly reached by the aluminum tripod than by any other. It should
have not less than three joints in order to facilitate the use of the long focus
upon objects near the ground.
Fig. 58. 5 × 7 “Korona” Royal camera.
237. Choice of a camera. There is not a great deal of choice between the
moderate-priced cameras of the various makers. A field camera is restricted
to certain special uses, and hence is more serviceable when attachments
capable of being stopped down to 128 or 256. The shutters furnished with
the ordinary lenses are satisfactory, since “snap-shots,” i. e., instantaneous
exposures, are practically never possible for plants. The automatic shutter of
the “Premo” camera is an especially convenient form. All shutters should be
carefully tested before using to determine the exact time value of the
exposures indicated. It is not uncommon for the exposure at 1 second, or at
other points, to have a value quite different from the one indicated. When
this is the case, it is evident that it can not be known too soon. The camera
should have at least a half-dozen double plate-holders. These are numbered
consecutively so that the figure uppermost when the holder is in the camera
will indicate the number of the plate exposed. A carrying case is desirable
on a long trip when all the plate-holders must be taken, but ordinarily it is a
disadvantage, since the camera box will carry two or three holders. The
camera cloth should be as small and light as possible, and at the same time
opaque. The most satisfactory one for the field is the rubber cloth. The
tripod should be a happy combination of lightness and stability, a condition
more nearly reached by the aluminum tripod than by any other. It should
have not less than three joints in order to facilitate the use of the long focus
upon objects near the ground.
Fig. 58. 5 × 7 “Korona” Royal camera.
237. Choice of a camera. There is not a great deal of choice between the
moderate-priced cameras of the various makers. A field camera is restricted
to certain special uses, and hence is more serviceable when attachments
Page 251
useful only in portraiture or instantaneous work are absent. Even the ray
filter, which has some value in the indoor photography of flowers, is useless
in the field on account of the long exposure required. From considerable
experience, “Premo” and “Korona” cameras have been found to be very
satisfactory instruments. Doubtless the same statement would be found true
of all the standard makes, but they have not been used by the writer.
“Premo” cameras are made by the Rochester Optical Co., Rochester, N. Y.,
and “Korona” cameras by the Gundlach-Manhattan Optical Co., Rochester,
N. Y. When two or more cameras are used, the best results can be obtained
if they are of the same make, since the details of operation are then the
same. The reduced liability of making a blunder is often offset by the fact
that a different pattern will permit of a wider range of use. Any standard
brand of plates will produce good negatives when skilfully used; at least,
this has been proved in the case of the Cramer, Hammer, Seed, and Stanley
brands. Every professional photographer has his favorite brand of plate, but
the ecologist will do well to give the various kinds a thorough trial, and then
to invariably use the one which gives him the best results. Thus, while it
seems to be less popular with the profession than the others mentioned, the
writer has obtained at least as satisfactory results with the Stanley plate as
with the others, and consequently now uses it exclusively, since it is
cheaper. The one important point is to make a final choice only after
personal experience, and then to always use plates of the same brand, and
preferably of the same rate of speed.
238. The use of the camera. To the ecologist, objects to be photographed
fall into two categories, viz., those that move, and those that do not move.
For practical purposes, areas sufficiently distant to render the movement
imperceptible belong to the latter, as well as those, such as rock lichens,
many fungi, etc., which can not be stirred by ordinary winds. The treatment
accorded the two is essentially different. A fundamental rule of ecological
photography is that detail must receive the first emphasis. The ecological
view should be a picture as well as a map, however, but when one must be
sacrificed, artistic effect must yield to clearness, and accuracy, i. e.,
technically speaking, contrast must give way to detail. Leaving apart the
necessity of securing a sharp focus, which holds for all work, detail or
definition depends directly upon the aperture of the diaphragm. Detail is
increased by decreasing the size of the aperture. This in turn increases the
length of time necessary for a proper exposure, and consequently the danger
filter, which has some value in the indoor photography of flowers, is useless
in the field on account of the long exposure required. From considerable
experience, “Premo” and “Korona” cameras have been found to be very
satisfactory instruments. Doubtless the same statement would be found true
of all the standard makes, but they have not been used by the writer.
“Premo” cameras are made by the Rochester Optical Co., Rochester, N. Y.,
and “Korona” cameras by the Gundlach-Manhattan Optical Co., Rochester,
N. Y. When two or more cameras are used, the best results can be obtained
if they are of the same make, since the details of operation are then the
same. The reduced liability of making a blunder is often offset by the fact
that a different pattern will permit of a wider range of use. Any standard
brand of plates will produce good negatives when skilfully used; at least,
this has been proved in the case of the Cramer, Hammer, Seed, and Stanley
brands. Every professional photographer has his favorite brand of plate, but
the ecologist will do well to give the various kinds a thorough trial, and then
to invariably use the one which gives him the best results. Thus, while it
seems to be less popular with the profession than the others mentioned, the
writer has obtained at least as satisfactory results with the Stanley plate as
with the others, and consequently now uses it exclusively, since it is
cheaper. The one important point is to make a final choice only after
personal experience, and then to always use plates of the same brand, and
preferably of the same rate of speed.
238. The use of the camera. To the ecologist, objects to be photographed
fall into two categories, viz., those that move, and those that do not move.
For practical purposes, areas sufficiently distant to render the movement
imperceptible belong to the latter, as well as those, such as rock lichens,
many fungi, etc., which can not be stirred by ordinary winds. The treatment
accorded the two is essentially different. A fundamental rule of ecological
photography is that detail must receive the first emphasis. The ecological
view should be a picture as well as a map, however, but when one must be
sacrificed, artistic effect must yield to clearness, and accuracy, i. e.,
technically speaking, contrast must give way to detail. Leaving apart the
necessity of securing a sharp focus, which holds for all work, detail or
definition depends directly upon the aperture of the diaphragm. Detail is
increased by decreasing the size of the aperture. This in turn increases the
length of time necessary for a proper exposure, and consequently the danger
Page 252
that the plant will be moved in the midst of the exposure. When the
movement is negligible, the invariable rule should be to reduce the aperture
to its smallest size, and to expose for a corresponding time. In all cases
where the plants are close enough to show even a slight blurring on account
of the action of the wind, the time of exposure must be reduced, in the hope
that a short period of quiet will suffice for it. This reduction in time must be
compensated by increasing the aperture of the diaphragm, and hence the
amount of light which strikes the plate. The proper balance between the two
is a matter of considerable nicety. It depends much upon the vagaries of the
wind, and can readily be determined only after considerable experience.
Although regions naturally differ somewhat in the nature of their winds,
much experience in prairie and mountain regions warrants the primary rule
that views of vegetation and plants subject to movement are not to be
attempted on windy or cloudy days when it can possibly be avoided. Even
on reconnaissance, a poor picture is no better than none at all, while in
resident work a time will come sooner or later which will permit the making
of a view satisfactory in all respects. There may be occasional instances
when one is rewarded for keeping the camera trained on a particular spot
for hours, and for wasting several plates in the hope that still moments will
prove to be of the requisite duration. As a regular procedure, however, this
has nothing to commend it.
Various methods have been tried to reduce or eliminate the trouble
caused by the wind. Canvas screens have been used for this purpose with
some benefit. When the picture is worth the trouble, a tent may be erected
to afford a very efficient protection. This is too prodigal of time and energy,
however, to be practicable under the usual conditions. Flashlight exposures
on still nights are sometimes feasible, but the disadvantages connected with
them are too great to bring them into general use. The best procedure is to
bide one’s time, and to take quadrats, transects, and other detail areas, as
well as many plant groups, at a time that promises to be most favorable.
Single plants can often be moved in the field so that they are protected from
the wind, or so that they are more strongly lighted. Slender, or feathery
plants are usually very difficult to handle out of doors. The best plan is to
photograph them in a room that is well and evenly lighted, or, best of all, in
a stable, roomy tent.
239. The sequence of details. No photographer ever escapes blunders
entirely. At the outset of his work, the ecologist must fully realize this, and
movement is negligible, the invariable rule should be to reduce the aperture
to its smallest size, and to expose for a corresponding time. In all cases
where the plants are close enough to show even a slight blurring on account
of the action of the wind, the time of exposure must be reduced, in the hope
that a short period of quiet will suffice for it. This reduction in time must be
compensated by increasing the aperture of the diaphragm, and hence the
amount of light which strikes the plate. The proper balance between the two
is a matter of considerable nicety. It depends much upon the vagaries of the
wind, and can readily be determined only after considerable experience.
Although regions naturally differ somewhat in the nature of their winds,
much experience in prairie and mountain regions warrants the primary rule
that views of vegetation and plants subject to movement are not to be
attempted on windy or cloudy days when it can possibly be avoided. Even
on reconnaissance, a poor picture is no better than none at all, while in
resident work a time will come sooner or later which will permit the making
of a view satisfactory in all respects. There may be occasional instances
when one is rewarded for keeping the camera trained on a particular spot
for hours, and for wasting several plates in the hope that still moments will
prove to be of the requisite duration. As a regular procedure, however, this
has nothing to commend it.
Various methods have been tried to reduce or eliminate the trouble
caused by the wind. Canvas screens have been used for this purpose with
some benefit. When the picture is worth the trouble, a tent may be erected
to afford a very efficient protection. This is too prodigal of time and energy,
however, to be practicable under the usual conditions. Flashlight exposures
on still nights are sometimes feasible, but the disadvantages connected with
them are too great to bring them into general use. The best procedure is to
bide one’s time, and to take quadrats, transects, and other detail areas, as
well as many plant groups, at a time that promises to be most favorable.
Single plants can often be moved in the field so that they are protected from
the wind, or so that they are more strongly lighted. Slender, or feathery
plants are usually very difficult to handle out of doors. The best plan is to
photograph them in a room that is well and evenly lighted, or, best of all, in
a stable, roomy tent.
239. The sequence of details. No photographer ever escapes blunders
entirely. At the outset of his work, the ecologist must fully realize this, and
Page 253
accordingly plan a method of operating the camera which will reduce the
chance of mistake to a minimum. The usual blunders which every one
makes sooner or later, such as making two exposures on one plate, drawing
the slide before closing the shutter, allowing the light to strike the plate
through the slit in the holder, etc., can be all but absolutely avoided by a
fixed order of doing things. This order will naturally not be the same for
different persons; it is necessary merely that each have his own invariable
sequence. The following one will serve as an illustration. As a preliminary,
the plate-holders are filled, after having been carefully dusted, and the
slides are uniformly replaced with the black edge inward. It is a wise
precaution to again see that all the slides are in this position before leaving
the dark room. This will ensure that a black edge outward always means
that the plate has been exposed. The tripod is first set up and placed in what
seems about the proper position. The camera is next attached to it, and the
front and back opened. The bellows is pulled out, a short distance for views,
and a longer one for detail pictures, and fastened. It is necessary to move
the diaphragm index to the largest aperture and to open the shutter at
“time.” The next steps are to orient the view or object, and to bring it into
sharp focus upon the ground glass. The first is accomplished by moving the
entire instrument, changing the position of the tripod legs, swinging the
camera upon the tripod, or by raising or lowering the lens front. It is often
desirable also to change the position of the object on the plate by use of the
reversible back. In views with much distance, the foreground is brought into
sharp focus. In close views, especially of quadrats, the swing is used to
increase the distance for the foreground, and the focus is made upon the
center. After focusing, the shutter is closed, the indicator set at the time
desired, and the diaphragm “stopped down” as far as possible. Plate-holder
1 is slipped into place, care being taken not to move the camera by a sudden
jar. The camera cloth is dropped above the holder and allowed to hang
down over the slide end. The slide is drawn and put on top of the
instrument, the black edge always up. The exposure is made and the slide
replaced with the black edge outward. This point should receive the most
critical attention, as a blunder here will often cause the loss of two
negatives. The plate-holder is returned to the receptacle, or merely placed in
the back of the camera, which is then closed. The number of the plate, the
name of the view or object, the condition of the light, the length of
exposure, and the aperture of the diaphragm, as well as the date, are
chance of mistake to a minimum. The usual blunders which every one
makes sooner or later, such as making two exposures on one plate, drawing
the slide before closing the shutter, allowing the light to strike the plate
through the slit in the holder, etc., can be all but absolutely avoided by a
fixed order of doing things. This order will naturally not be the same for
different persons; it is necessary merely that each have his own invariable
sequence. The following one will serve as an illustration. As a preliminary,
the plate-holders are filled, after having been carefully dusted, and the
slides are uniformly replaced with the black edge inward. It is a wise
precaution to again see that all the slides are in this position before leaving
the dark room. This will ensure that a black edge outward always means
that the plate has been exposed. The tripod is first set up and placed in what
seems about the proper position. The camera is next attached to it, and the
front and back opened. The bellows is pulled out, a short distance for views,
and a longer one for detail pictures, and fastened. It is necessary to move
the diaphragm index to the largest aperture and to open the shutter at
“time.” The next steps are to orient the view or object, and to bring it into
sharp focus upon the ground glass. The first is accomplished by moving the
entire instrument, changing the position of the tripod legs, swinging the
camera upon the tripod, or by raising or lowering the lens front. It is often
desirable also to change the position of the object on the plate by use of the
reversible back. In views with much distance, the foreground is brought into
sharp focus. In close views, especially of quadrats, the swing is used to
increase the distance for the foreground, and the focus is made upon the
center. After focusing, the shutter is closed, the indicator set at the time
desired, and the diaphragm “stopped down” as far as possible. Plate-holder
1 is slipped into place, care being taken not to move the camera by a sudden
jar. The camera cloth is dropped above the holder and allowed to hang
down over the slide end. The slide is drawn and put on top of the
instrument, the black edge always up. The exposure is made and the slide
replaced with the black edge outward. This point should receive the most
critical attention, as a blunder here will often cause the loss of two
negatives. The plate-holder is returned to the receptacle, or merely placed in
the back of the camera, which is then closed. The number of the plate, the
name of the view or object, the condition of the light, the length of
exposure, and the aperture of the diaphragm, as well as the date, are
Page 254
recorded in a notebook for this purpose. The shutter is then opened at
“time,” the diaphragm thrown wide open, and the front of the camera
closed. When distances are short, the camera is often carried upon the
tripod. As a rule, however, it is usually removed, and the tripod folded. In
making subsequent pictures, the plates should always be used in their
numerical order.
240. The time of exposure is obviously the most critical task in the
manipulation of a camera. The time necessary for a proper exposure varies
with the season, the hour, the condition of the sky, the light intensity of the
formation, the color and size of the area to be photographed, and, finally, of
course, with the aperture of the diaphragm. Fortunately for the ecologist,
the variation in light intensity during the season, and even during the greater
part of the day, is not great, and can ordinarily be ignored. The beginner
will make the most progress by determining the exposure demanded by his
instrument for taking a general view in full sunlight and with the smallest
stop of the diaphragm. In standard cameras with lenses of ordinary rapidity,
this is usually about one second. This will serve as a basis from which all
other exposures may be reckoned until one has worked through a wide
range of conditions and can recall just what time each view requires. On
completely cloudy days the time required is five to ten times that necessary
on a clear day; filmy clouds and haze necessitate an exposure of two or
three seconds. The more open forest formations demand an exposure of
about five to ten seconds on a sunny day, while the deeper ones require two
or three times as long. A close view requires more time than a distant one,
since the light-reflecting surface is much smaller. Quadrats require two or
three seconds, and individual groups frequently take a longer time. The
color of the vegetation plays an important part also: a dark green spruce
forest requires twice as long an exposure as the aspen forest, and a
grassland quadrat takes more time than one located in a gravel slide. In this
connection, it is hardly necessary to point out that the lighted side of objects
should always be taken, never the shaded one. The exposures indicated
above are based upon the smallest stop. The reasons for using this whenever
possible have already been given. When a larger stop is necessary, the
exposure is decreased to correspond; for example, a quadrat that takes three
to four seconds at 256 can be taken at 64 in one second. As a rule, the sun
should not be in front of the camera, but, when necessary, views can be
“time,” the diaphragm thrown wide open, and the front of the camera
closed. When distances are short, the camera is often carried upon the
tripod. As a rule, however, it is usually removed, and the tripod folded. In
making subsequent pictures, the plates should always be used in their
numerical order.
240. The time of exposure is obviously the most critical task in the
manipulation of a camera. The time necessary for a proper exposure varies
with the season, the hour, the condition of the sky, the light intensity of the
formation, the color and size of the area to be photographed, and, finally, of
course, with the aperture of the diaphragm. Fortunately for the ecologist,
the variation in light intensity during the season, and even during the greater
part of the day, is not great, and can ordinarily be ignored. The beginner
will make the most progress by determining the exposure demanded by his
instrument for taking a general view in full sunlight and with the smallest
stop of the diaphragm. In standard cameras with lenses of ordinary rapidity,
this is usually about one second. This will serve as a basis from which all
other exposures may be reckoned until one has worked through a wide
range of conditions and can recall just what time each view requires. On
completely cloudy days the time required is five to ten times that necessary
on a clear day; filmy clouds and haze necessitate an exposure of two or
three seconds. The more open forest formations demand an exposure of
about five to ten seconds on a sunny day, while the deeper ones require two
or three times as long. A close view requires more time than a distant one,
since the light-reflecting surface is much smaller. Quadrats require two or
three seconds, and individual groups frequently take a longer time. The
color of the vegetation plays an important part also: a dark green spruce
forest requires twice as long an exposure as the aspen forest, and a
grassland quadrat takes more time than one located in a gravel slide. In this
connection, it is hardly necessary to point out that the lighted side of objects
should always be taken, never the shaded one. The exposures indicated
above are based upon the smallest stop. The reasons for using this whenever
possible have already been given. When a larger stop is necessary, the
exposure is decreased to correspond; for example, a quadrat that takes three
to four seconds at 256 can be taken at 64 in one second. As a rule, the sun
should not be in front of the camera, but, when necessary, views can be
Page 255
made in this position if the sun is prevented from shining directly into the
lens.
241. Developing is as important as exposing. Indeed, it may well be
considered more important, since a properly exposed plate may be spoiled
in developing, while an under-exposure or over-exposure may be saved.
Owing to the ease with which plants move in the wind, the ecologist is
obliged to reconcile himself to many under-exposures, which can be
converted into good negatives only by skilful developing. Every base
station should have a good dark room, equipped with running water when
possible, a good ruby lantern, and the proper trays and chemicals. Prepared
developing solutions are alluring because of their convenience, but after an
extended trial of several kinds, the writer has reached the conviction that
pyrogallic acid, or “pyro,” is by far the most satisfactory in working with
vegetation. Of almost innumerable formulae, the following gives excellent
satisfaction and is convenient to use.
I. II.
500 cc. water 500 cc. water
30 grams sodium sulphite 5 grams pyrogallic acid
30 grams sodium carbonate
For developing, equal parts of I and II are mixed, and a few drops of a 10
per cent solution of potassium bromide added, unless there is reason to
suspect that the plate has been seriously underexposed. The fixing bath is a
concentrated solution of sodium hyposulphite, “hypo,” to which a few
drops of acetic acid are added. It should be replaced every week or two,
depending upon how much it is used. A tray of water is kept at hand for
bringing out the detail in underexposed negatives, and a second tray is used
for washing. The “pyro” and the bromide solution should always be within
reach, the former for accelerating, and the latter for retarding the
development of unsatisfactory plates.
The image will begin to show on a properly exposed plate within one to
three minutes after it has been put in the developer. If the image appears
almost instantly, and then recedes quickly, the plate is badly overexposed,
and should be thrown away. In case it “comes up” less quickly, indicating
that it is not greatly overexposed, it can be saved by the addition of more
lens.
241. Developing is as important as exposing. Indeed, it may well be
considered more important, since a properly exposed plate may be spoiled
in developing, while an under-exposure or over-exposure may be saved.
Owing to the ease with which plants move in the wind, the ecologist is
obliged to reconcile himself to many under-exposures, which can be
converted into good negatives only by skilful developing. Every base
station should have a good dark room, equipped with running water when
possible, a good ruby lantern, and the proper trays and chemicals. Prepared
developing solutions are alluring because of their convenience, but after an
extended trial of several kinds, the writer has reached the conviction that
pyrogallic acid, or “pyro,” is by far the most satisfactory in working with
vegetation. Of almost innumerable formulae, the following gives excellent
satisfaction and is convenient to use.
I. II.
500 cc. water 500 cc. water
30 grams sodium sulphite 5 grams pyrogallic acid
30 grams sodium carbonate
For developing, equal parts of I and II are mixed, and a few drops of a 10
per cent solution of potassium bromide added, unless there is reason to
suspect that the plate has been seriously underexposed. The fixing bath is a
concentrated solution of sodium hyposulphite, “hypo,” to which a few
drops of acetic acid are added. It should be replaced every week or two,
depending upon how much it is used. A tray of water is kept at hand for
bringing out the detail in underexposed negatives, and a second tray is used
for washing. The “pyro” and the bromide solution should always be within
reach, the former for accelerating, and the latter for retarding the
development of unsatisfactory plates.
The image will begin to show on a properly exposed plate within one to
three minutes after it has been put in the developer. If the image appears
almost instantly, and then recedes quickly, the plate is badly overexposed,
and should be thrown away. In case it “comes up” less quickly, indicating
that it is not greatly overexposed, it can be saved by the addition of more
Page 256
bromide. When the image does not show till the end of five to ten minutes,
the plate has been underexposed. It is then necessary to add more “pyro,”
taking care not to pour it on the plate, and, after the image appears with its
striking contrast, to leave the plate in water until as much detail as possible
is brought out in the shadows. In the case of a normal exposure, when
greater detail is desired, the negative is left for some time in water, and
when contrast is sought more “pyro” is used. Negatives with unusual detail
lack “snap”; they are “flat,” and fail to make artistic pictures. Contrast, on
the other hand, often obscures detail, and the best results can only be
obtained by a happy combination of the two. The most important maxim in
developing is that the process shall be continued until the image has
become indistinct. The universal tendency of the beginner is to remove the
negative the moment the outlines grow dimmer, and the result is a thin,
lifeless negative. It is almost impossible to develop too far, if the image is
not allowed to disappear. Negatives of this sort are “thick,” and though they
print more slowly, produce brilliant pictures. A large quantity of the
developing solution is used with single plates in small trays, and is allowed
to act without rocking the tray. Much time is saved, however, by developing
several plates together, and to avoid using a large quantity of the solution,
the tray is gently rocked from time to time. This movement is particularly
necessary at the beginning, in order that the plates may be covered evenly,
and at once. Fifty cubic centimeters of the solution will develop three or
four 6½ × 8½ plates, and twice as many 4 × 5’s. After the developer has
once been used, it is kept for several days to restrain overexposed plates. As
soon as the plate is developed, it is rinsed in water, and placed in the fixing
fluid, until the white opaqueness is entirely removed. The “hypo” is then
washed out by immersing the negatives for one to two hours in running
water. If the latter can not be secured, the water in which they are placed
should be changed frequently. The negatives are then air-dried within doors,
in a place free from dust. Finally, they are filed away in negative envelopes,
each bearing the name and number of the negative, and preferably also, the
time and other exposure data.
242. Finishing. On account of the time demanded by other field tasks, it
has not been found desirable to make and finish prints in the field. This,
with the making of lantern slides, enlargements, etc., may well be turned
over to a professional photographer. It is the custom to make a proof of each
negative to meet the casual needs that arise in the field. For this purpose,
the plate has been underexposed. It is then necessary to add more “pyro,”
taking care not to pour it on the plate, and, after the image appears with its
striking contrast, to leave the plate in water until as much detail as possible
is brought out in the shadows. In the case of a normal exposure, when
greater detail is desired, the negative is left for some time in water, and
when contrast is sought more “pyro” is used. Negatives with unusual detail
lack “snap”; they are “flat,” and fail to make artistic pictures. Contrast, on
the other hand, often obscures detail, and the best results can only be
obtained by a happy combination of the two. The most important maxim in
developing is that the process shall be continued until the image has
become indistinct. The universal tendency of the beginner is to remove the
negative the moment the outlines grow dimmer, and the result is a thin,
lifeless negative. It is almost impossible to develop too far, if the image is
not allowed to disappear. Negatives of this sort are “thick,” and though they
print more slowly, produce brilliant pictures. A large quantity of the
developing solution is used with single plates in small trays, and is allowed
to act without rocking the tray. Much time is saved, however, by developing
several plates together, and to avoid using a large quantity of the solution,
the tray is gently rocked from time to time. This movement is particularly
necessary at the beginning, in order that the plates may be covered evenly,
and at once. Fifty cubic centimeters of the solution will develop three or
four 6½ × 8½ plates, and twice as many 4 × 5’s. After the developer has
once been used, it is kept for several days to restrain overexposed plates. As
soon as the plate is developed, it is rinsed in water, and placed in the fixing
fluid, until the white opaqueness is entirely removed. The “hypo” is then
washed out by immersing the negatives for one to two hours in running
water. If the latter can not be secured, the water in which they are placed
should be changed frequently. The negatives are then air-dried within doors,
in a place free from dust. Finally, they are filed away in negative envelopes,
each bearing the name and number of the negative, and preferably also, the
time and other exposure data.
242. Finishing. On account of the time demanded by other field tasks, it
has not been found desirable to make and finish prints in the field. This,
with the making of lantern slides, enlargements, etc., may well be turned
over to a professional photographer. It is the custom to make a proof of each
negative to meet the casual needs that arise in the field. For this purpose,
Page 257
solio “seconds” are used, since they are both cheap and satisfactory. When
an urgent demand for a finished print does arise, it is met by using “velox”
paper, which can be exposed in the dark room, and then developed and
fixed exactly like a plate. Two standard papers for views are “solio” and
“platina.” The former gives brown tones, and is used for contrast and
brilliancy, hence it is especially good for printing from negatives that have
too much detail and too little contrast. “Platina,” on the contrary, yields soft
gray tones, and softens contrasts.
an urgent demand for a finished print does arise, it is met by using “velox”
paper, which can be exposed in the dark room, and then developed and
fixed exactly like a plate. Two standard papers for views are “solio” and
“platina.” The former gives brown tones, and is used for contrast and
brilliancy, hence it is especially good for printing from negatives that have
too much detail and too little contrast. “Platina,” on the contrary, yields soft
gray tones, and softens contrasts.
Page 258
FORMATION AND SUCCESSION HERBARIA
243. Concept and purpose. A formation herbarium is a collection of
exsiccati, in which the species are arranged with respect to their position in
the formation, instead of being grouped in genera and families. Its primary
purpose is to furnish a record of the constitution and the structure of a
formation or a series of formations. At the same time, it affords the basal
material for developing the subject of comparative phytogeography. It is
impossible for one ecologist to visit many remote regions, to say nothing of
spending a period sufficient for obtaining even a fair knowledge of the
vegetation. He can at the best acquire an acquaintance with but few regions
at first hand. In consequence, a method that brings a vegetation to him, with
its structure carefully wrought out by years of study, is of the highest value.
Time, as well as distance, sets a narrow limit to the number of formations
which one man can investigate critically in a lifetime. It is no longer
possible for a botanist to explore vast regions, and to bring back results
which have anything more than a very general value. This fact, far from
restricting the comparative study of vegetation, will serve to make it more
accurate and systematic. The exact results of numerous resident
investigators, expressed in formation herbaria, with the proper series of
quadrat maps and photographs, will be worked over by men who are
themselves specially acquainted with a particular vegetation. Comparisons
will be founded upon a definite basis, and the relationship of various
vegetations can then be expressed in precise rather than general terms. It is
hardly too sweeping to assert that accurate work in the field of comparative
phytogeography can be done only in this fashion. The value of formation
herbaria in class work is evident. On account of the limitations of time and
distance, classes can touch but few formations, and these at every time
except the growing period. For these reasons, an accurate and complete
formational record that can be consulted or studied at any time is almost
indispensable to class study in the development and structure of formations.
244. Details of collecting. Formational collections, unlike the ordinary
sets of exsiccati, can not be made upon the first visit to a region, or by a
single journey through it. The determination of formation limits, and of
developmental stages, of aspects, layers, abundance, etc., must necessarily
243. Concept and purpose. A formation herbarium is a collection of
exsiccati, in which the species are arranged with respect to their position in
the formation, instead of being grouped in genera and families. Its primary
purpose is to furnish a record of the constitution and the structure of a
formation or a series of formations. At the same time, it affords the basal
material for developing the subject of comparative phytogeography. It is
impossible for one ecologist to visit many remote regions, to say nothing of
spending a period sufficient for obtaining even a fair knowledge of the
vegetation. He can at the best acquire an acquaintance with but few regions
at first hand. In consequence, a method that brings a vegetation to him, with
its structure carefully wrought out by years of study, is of the highest value.
Time, as well as distance, sets a narrow limit to the number of formations
which one man can investigate critically in a lifetime. It is no longer
possible for a botanist to explore vast regions, and to bring back results
which have anything more than a very general value. This fact, far from
restricting the comparative study of vegetation, will serve to make it more
accurate and systematic. The exact results of numerous resident
investigators, expressed in formation herbaria, with the proper series of
quadrat maps and photographs, will be worked over by men who are
themselves specially acquainted with a particular vegetation. Comparisons
will be founded upon a definite basis, and the relationship of various
vegetations can then be expressed in precise rather than general terms. It is
hardly too sweeping to assert that accurate work in the field of comparative
phytogeography can be done only in this fashion. The value of formation
herbaria in class work is evident. On account of the limitations of time and
distance, classes can touch but few formations, and these at every time
except the growing period. For these reasons, an accurate and complete
formational record that can be consulted or studied at any time is almost
indispensable to class study in the development and structure of formations.
244. Details of collecting. Formational collections, unlike the ordinary
sets of exsiccati, can not be made upon the first visit to a region, or by a
single journey through it. The determination of formation limits, and of
developmental stages, of aspects, layers, abundance, etc., must necessarily
Page 259
precede, a work which alone takes several years. Moreover, collecting itself
requires more than one year in a region containing numerous formations.
This is exemplified by the Herbaria Formationum Coloradensium.[23] The
preliminary study for this was made from 1896–1899, the collecting was
done chiefly in 1900 and 1901, while additional numbers were added in
1902–3. For the purposes of the formation herbarium, specimens should be
collected and pressed in such fashion as to show all the ecological features
possible. Plants must be collected both in flower and in fruit, with the
underground parts as perfect as may be. Seedlings and rosettes should be
included whenever present. In pressing, one or two leaves should be
arranged with the lower side uppermost to admit of the ready comparison of
both surfaces. Opened flowers are valuable for flower biology, while seeds
and fruits are desirable for showing migration contrivances. The ferns,
mosses, and lichens of the formation should be fully represented, together
with the more important fungi and algae. The number of photographs taken
for each herbarium should be limited only by considerations of time and
expense. The ideal series consists of a general view of each formation,
showing its physiographic setting, nearer views of each of its aspects, detail
views of its consocies, societies, and layers, and flower portraits of all the
constituent species. Such a series can only be obtained by residence through
a long term of years, and in most cases general and aspect views, with
portraits of the facies and a few of the striking principal species, must
suffice. Quadrat and transect charts, together with formational maps, are
extremely desirable, and, indeed, all but indispensable.
245. Arrangement. The arrangement of species within each formation
herbarium is based upon the structure of the vegetation. The primary
groupings are made with reference to time of appearance and abundance;
when definite zones, associations, or layers are present, they must likewise
be taken into account. In the Colorado collection, the first division is into
three aspects based upon the period of flowering (aspectus vernalis,
aestivalis, autumnalis). Within each aspect, the species are arranged with
respect to abundance in the groups, facies, principal species, and secondary
species. Each group is placed in an ordinary manila cover, which bears a
printed label indicating the aspect and the group. The species labels give, in
addition to the name, date, and place of collection, the phyad or vegetation
form, the geographical area, the rank of the species, the aspect, and the
formation. To these may well be added data concerning migration
requires more than one year in a region containing numerous formations.
This is exemplified by the Herbaria Formationum Coloradensium.[23] The
preliminary study for this was made from 1896–1899, the collecting was
done chiefly in 1900 and 1901, while additional numbers were added in
1902–3. For the purposes of the formation herbarium, specimens should be
collected and pressed in such fashion as to show all the ecological features
possible. Plants must be collected both in flower and in fruit, with the
underground parts as perfect as may be. Seedlings and rosettes should be
included whenever present. In pressing, one or two leaves should be
arranged with the lower side uppermost to admit of the ready comparison of
both surfaces. Opened flowers are valuable for flower biology, while seeds
and fruits are desirable for showing migration contrivances. The ferns,
mosses, and lichens of the formation should be fully represented, together
with the more important fungi and algae. The number of photographs taken
for each herbarium should be limited only by considerations of time and
expense. The ideal series consists of a general view of each formation,
showing its physiographic setting, nearer views of each of its aspects, detail
views of its consocies, societies, and layers, and flower portraits of all the
constituent species. Such a series can only be obtained by residence through
a long term of years, and in most cases general and aspect views, with
portraits of the facies and a few of the striking principal species, must
suffice. Quadrat and transect charts, together with formational maps, are
extremely desirable, and, indeed, all but indispensable.
245. Arrangement. The arrangement of species within each formation
herbarium is based upon the structure of the vegetation. The primary
groupings are made with reference to time of appearance and abundance;
when definite zones, associations, or layers are present, they must likewise
be taken into account. In the Colorado collection, the first division is into
three aspects based upon the period of flowering (aspectus vernalis,
aestivalis, autumnalis). Within each aspect, the species are arranged with
respect to abundance in the groups, facies, principal species, and secondary
species. Each group is placed in an ordinary manila cover, which bears a
printed label indicating the aspect and the group. The species labels give, in
addition to the name, date, and place of collection, the phyad or vegetation
form, the geographical area, the rank of the species, the aspect, and the
formation. To these may well be added data concerning migration
Page 260
contrivances, seed production, pollination, period of flowering, etc. The
photographs are mounted on the usual herbarium sheets, and placed in the
proper order in the various groups, and a similar disposition is made of
quadrat and transect charts, and such physical factor summaries as seem
desirable.
246. Succession herbaria. The arrangement of formation herbaria may
follow the classification of formations with respect to character, region, or
development. The first is the most convenient for purposes of instruction,
and has distinct advantages in permitting a close comparison of the
vegetation of different habitats. The second basis, which is the one used in
the Herbaria Formationum Coloradensium, is peculiarly adapted to
mountain vegetation in which the zones are usually very distinct. The
arrangement of herbaria in a developmental series, however, is the most
logical and the most illuminating, since the structure of the ultimate
formations is not only made plain, but the stages in their development are
also laid bare. Such succession herbaria are the natural outgrowth of
formational ones. Indeed, the latter should be made merely the starting
point for these in all regions where the causes which bring about
successions are active. Where weathering is still an important factor, as in
mountains, the initial and intermediate formations which lead to the final
grassland or forest are often in evidence. After a formation herbarium of
each stage has been made in the way indicated, a succession herbarium is
obtained merely by arranging the various herbaria in the sequence of the
developmental stages. Thus, in the Colorado collection, the subalpine
formations are arranged according to altitude in the following series: (1) the
pine formation, (2) the gravel slide formation, (3) the half gravel slide
formation, (4) the aspen formation, (5) the balsam-spruce formation, (6) the
spruce-pine formation, (7) the meadow thicket formation, (8) the brook
bank formation. Of these, five belong to the same succession, and it is
possible to indicate the development of the spruce-pine forest by arranging
these five formations in their proper order in a succession herbarium, as
follows: (1) the gravel slide formation, (2) the half gravel slide formation,
(3) the pine formation, (4) the balsam-spruce formation, (5) the spruce-pine
formation.
Development and Structure
photographs are mounted on the usual herbarium sheets, and placed in the
proper order in the various groups, and a similar disposition is made of
quadrat and transect charts, and such physical factor summaries as seem
desirable.
246. Succession herbaria. The arrangement of formation herbaria may
follow the classification of formations with respect to character, region, or
development. The first is the most convenient for purposes of instruction,
and has distinct advantages in permitting a close comparison of the
vegetation of different habitats. The second basis, which is the one used in
the Herbaria Formationum Coloradensium, is peculiarly adapted to
mountain vegetation in which the zones are usually very distinct. The
arrangement of herbaria in a developmental series, however, is the most
logical and the most illuminating, since the structure of the ultimate
formations is not only made plain, but the stages in their development are
also laid bare. Such succession herbaria are the natural outgrowth of
formational ones. Indeed, the latter should be made merely the starting
point for these in all regions where the causes which bring about
successions are active. Where weathering is still an important factor, as in
mountains, the initial and intermediate formations which lead to the final
grassland or forest are often in evidence. After a formation herbarium of
each stage has been made in the way indicated, a succession herbarium is
obtained merely by arranging the various herbaria in the sequence of the
developmental stages. Thus, in the Colorado collection, the subalpine
formations are arranged according to altitude in the following series: (1) the
pine formation, (2) the gravel slide formation, (3) the half gravel slide
formation, (4) the aspen formation, (5) the balsam-spruce formation, (6) the
spruce-pine formation, (7) the meadow thicket formation, (8) the brook
bank formation. Of these, five belong to the same succession, and it is
possible to indicate the development of the spruce-pine forest by arranging
these five formations in their proper order in a succession herbarium, as
follows: (1) the gravel slide formation, (2) the half gravel slide formation,
(3) the pine formation, (4) the balsam-spruce formation, (5) the spruce-pine
formation.
Development and Structure
Page 261
247. Vegetation an organism. The plant formation is an organic unit. It
exhibits activities or changes which result in development, structure, and
reproduction. These changes are progressive, or periodic, and, in some
degree, rhythmic, and there can be no objection to regarding them as
functions of vegetation. According to this point of view, the formation is a
complex organism, which possesses functions and structure, and passes
through a cycle of development similar to that of the plant. This concept
may seem strange at first, owing to the fact that the common understanding
of function and structure is based upon the individual plant alone. Since the
formation, like the plant, is subject to changes caused by the habitat, and
since these changes are recorded in its structure, it is evident that the terms,
function and structure, are as applicable to the one as to the other. It is
merely necessary to bear in mind that the functions of plants and of
formations are absolutely different activities, which have no more in
common than do the two structures, leaf and zone.
248. Vegetation essentially dynamic. As an organism, the formation is
undergoing constant change. Constructive or destructive forces are
necessarily at work; the former, as in the plant, predominate until maturity,
when the latter prevail. Consequently, it no longer seems fruitful to classify
the phenomena of vegetation as dynamic or static. The emphasis which has
been placed upon dynamic aspects of vegetation has served a useful
purpose by calling attention to the development of the latter. Although it is a
quarter of a century since Hult, and more than a half century since
Steenstrup, by far the greater number of ecological studies still ignore the
problem of development. This condition, however, can be remedied more
easily by insisting upon an exact understanding of the nature of the
formation than in any other way. It is entirely superfluous to speak of
dynamic and static effects in the plant, and the use of these terms with
reference to the formation becomes equally unnecessary as soon as the
latter is looked upon as an organism. The proper investigation of a
formation can no more overlook development than structure, so closely are
the two interwoven. Future research must rest squarely upon this fact.
249. Functions and structures. The functions of a formation are
association, invasion, and succession: the second may be resolved into
migration and ecesis, and the third, perhaps, into reaction and competition.
Formational structures comprise zones, layers, consocies, societies, etc., all
of which may be referred to zonation, or to alternation. The term association
exhibits activities or changes which result in development, structure, and
reproduction. These changes are progressive, or periodic, and, in some
degree, rhythmic, and there can be no objection to regarding them as
functions of vegetation. According to this point of view, the formation is a
complex organism, which possesses functions and structure, and passes
through a cycle of development similar to that of the plant. This concept
may seem strange at first, owing to the fact that the common understanding
of function and structure is based upon the individual plant alone. Since the
formation, like the plant, is subject to changes caused by the habitat, and
since these changes are recorded in its structure, it is evident that the terms,
function and structure, are as applicable to the one as to the other. It is
merely necessary to bear in mind that the functions of plants and of
formations are absolutely different activities, which have no more in
common than do the two structures, leaf and zone.
248. Vegetation essentially dynamic. As an organism, the formation is
undergoing constant change. Constructive or destructive forces are
necessarily at work; the former, as in the plant, predominate until maturity,
when the latter prevail. Consequently, it no longer seems fruitful to classify
the phenomena of vegetation as dynamic or static. The emphasis which has
been placed upon dynamic aspects of vegetation has served a useful
purpose by calling attention to the development of the latter. Although it is a
quarter of a century since Hult, and more than a half century since
Steenstrup, by far the greater number of ecological studies still ignore the
problem of development. This condition, however, can be remedied more
easily by insisting upon an exact understanding of the nature of the
formation than in any other way. It is entirely superfluous to speak of
dynamic and static effects in the plant, and the use of these terms with
reference to the formation becomes equally unnecessary as soon as the
latter is looked upon as an organism. The proper investigation of a
formation can no more overlook development than structure, so closely are
the two interwoven. Future research must rest squarely upon this fact.
249. Functions and structures. The functions of a formation are
association, invasion, and succession: the second may be resolved into
migration and ecesis, and the third, perhaps, into reaction and competition.
Formational structures comprise zones, layers, consocies, societies, etc., all
of which may be referred to zonation, or to alternation. The term association
Page 262
has been used in both an active and a passive sense. In the former, it applies
to the inevitable grouping together of plants, by means of reproduction and
immobility. Passively, it refers to the actual groupings which result in this
way, and in this sense it is practically synonymous with vegetation.
Invasion is the function of movement, and of occupying or taking
possession; with association, it constitutes the two fundamental activities of
vegetation. It is the essential part of succession, but the latter is so
distinctive, because of the intimate relation of competition and reaction, that
clearness is gained by treating it as a separate function which is especially
concerned with development. Association, zonation, and alternation are
structural phenomena, which are in large part the immediate product of
habitat and function, and in a considerable degree, also, the result of
ancestral or historical facts. It is a difficult matter to determine in what
measure the last factor enters, but it is one that must always be taken into
account, particularly when the physical factors of the habitat are inadequate
to explain the structures observed. Structurally, association regularly
includes both zonation and alternation. As there are certain typical instances
in which it exhibits neither, the treatment will be clearer if each is
considered separately.
to the inevitable grouping together of plants, by means of reproduction and
immobility. Passively, it refers to the actual groupings which result in this
way, and in this sense it is practically synonymous with vegetation.
Invasion is the function of movement, and of occupying or taking
possession; with association, it constitutes the two fundamental activities of
vegetation. It is the essential part of succession, but the latter is so
distinctive, because of the intimate relation of competition and reaction, that
clearness is gained by treating it as a separate function which is especially
concerned with development. Association, zonation, and alternation are
structural phenomena, which are in large part the immediate product of
habitat and function, and in a considerable degree, also, the result of
ancestral or historical facts. It is a difficult matter to determine in what
measure the last factor enters, but it is one that must always be taken into
account, particularly when the physical factors of the habitat are inadequate
to explain the structures observed. Structurally, association regularly
includes both zonation and alternation. As there are certain typical instances
in which it exhibits neither, the treatment will be clearer if each is
considered separately.
Page 263
ASSOCIATION
250. Concept. The principle of association is the fundamental law of
vegetation. Indeed, association is vegetation, for the individual passes into
vegetation, strictly speaking, at the moment when other individuals of the
same kind or of different kinds become grouped with it. It is then (and the
same statement necessarily holds for vegetation) the coming together and
the staying together of individuals and, ultimately, of species. A concrete
instance will illustrate this fact. In the development of the blowout
formation of the Nebraska sand-hills (Redfieldia-Muhlenbergia-anemium),
association begins only when the first plant of Redfieldia flexuosa is joined
by other plants that have sprung from it, or have wandered in over the
margin of the blowout. Henceforth, whatever changes the blowout
formation may undergo, association is a settled characteristic of it until
some new and overwhelming physical catastrophe shall destroy the
associated individuals. It will readily be seen that association does not
depend upon particular individuals, for these pass and others take their
place, but that it does depend essentially upon number of individuals.
Association involves the idea of the relation of plants to the soil, as well
as that of plants to each other. It is synonymous with vegetation only when
the two relations are represented, since there may be association such as
that of a parasite with its host, which does not constitute vegetation. But it
will be seen that the relation of the parasite to the host is practically
identical with the relation of the plant to the soil or stratum, and the two
concepts mentioned above become merged in such a case. From this it
follows that association results in vegetation only when the two ideas are
distinct. The concept of association contains a fact that is everywhere
significant of vegetation, namely, the likeness or unlikeness of the
individuals which are associated. In the case of parasite and host, this
unlikeness is marked; in vegetation, all degrees of similarity obtain. As will
be evident when the causes which lead to association are considered,
alternate similarity and dissimilarity of the constituent individuals or
species is subordinate as a feature of vegetation only to the primary fact of
association.
250. Concept. The principle of association is the fundamental law of
vegetation. Indeed, association is vegetation, for the individual passes into
vegetation, strictly speaking, at the moment when other individuals of the
same kind or of different kinds become grouped with it. It is then (and the
same statement necessarily holds for vegetation) the coming together and
the staying together of individuals and, ultimately, of species. A concrete
instance will illustrate this fact. In the development of the blowout
formation of the Nebraska sand-hills (Redfieldia-Muhlenbergia-anemium),
association begins only when the first plant of Redfieldia flexuosa is joined
by other plants that have sprung from it, or have wandered in over the
margin of the blowout. Henceforth, whatever changes the blowout
formation may undergo, association is a settled characteristic of it until
some new and overwhelming physical catastrophe shall destroy the
associated individuals. It will readily be seen that association does not
depend upon particular individuals, for these pass and others take their
place, but that it does depend essentially upon number of individuals.
Association involves the idea of the relation of plants to the soil, as well
as that of plants to each other. It is synonymous with vegetation only when
the two relations are represented, since there may be association such as
that of a parasite with its host, which does not constitute vegetation. But it
will be seen that the relation of the parasite to the host is practically
identical with the relation of the plant to the soil or stratum, and the two
concepts mentioned above become merged in such a case. From this it
follows that association results in vegetation only when the two ideas are
distinct. The concept of association contains a fact that is everywhere
significant of vegetation, namely, the likeness or unlikeness of the
individuals which are associated. In the case of parasite and host, this
unlikeness is marked; in vegetation, all degrees of similarity obtain. As will
be evident when the causes which lead to association are considered,
alternate similarity and dissimilarity of the constituent individuals or
species is subordinate as a feature of vegetation only to the primary fact of
association.
Page 264
Since association contains two distinct, though related, ideas, it is of
necessity ambiguous. It is very desirable that this be avoided, in order that
each concept may be clearly delimited. For this reason, the act or process of
grouping individuals is termed aggregation, while the word association is
restricted to the condition or state of being grouped together. In a word,
aggregation is functional, association is structural; the one is the result of
the other. This distinction makes clear the difference between association in
the active and passive sense, and falls in with the need of keeping function
and structure in the foreground.
251. Causes. In considering the causes which produce association, it is
necessary to call in evidence the primary facts of the process in concrete
examples of this principle. These facts are so bound up in the nature of
vegetal organisms that they are the veriest axioms. Reproduction gives rise
immediately to potential, and ultimately, in the great majority of cases, to
actual association. The degree and permanence of the association are then
determined by the immobility of the individuals as expressed in terms of
attachment to each other or to the stratum, such as sheath, thallus, haustoria,
holdfasts, rhizoids, roots, etc. The range of immobility is very great. In
terrestrial plants, mobility is confined almost entirely to the period when the
individual lies dormant in the seed, spore, or propagative part, which is
alone mobile. In aquatic spermatophytes, the same is true of all attached
forms, while free floating plants such as Lemna are mobile in a high degree,
especially during the vegetative period. Among the algae and hydrophilous
fungi, attached forms are mobile only in the spore or propagative condition,
while the motile forms of the plancton typify the extreme development of
mobility. The immediate result of reproduction in an immobile species is to
produce association of like individuals, while in the case of a mobile
species reproduction may or may not lead immediately to association. We
may lay down the general principle that immobility tends to maintain the
association of the individuals of the same generation, i. e., the association of
like forms, while mobility tends to separate the similar individuals of one
generation and to bring unlike forms together. With the mobile algae,
separation of the members of each generation is the rule, unless the
individuals come to be associated in a thallus, or are grouped in contact
with the substratum. Flowering plants that are relatively immobile,
especially in the seed state, drop their seeds beneath and about the parent
plants, and in consequence dense association of the new plants is the rule.
necessity ambiguous. It is very desirable that this be avoided, in order that
each concept may be clearly delimited. For this reason, the act or process of
grouping individuals is termed aggregation, while the word association is
restricted to the condition or state of being grouped together. In a word,
aggregation is functional, association is structural; the one is the result of
the other. This distinction makes clear the difference between association in
the active and passive sense, and falls in with the need of keeping function
and structure in the foreground.
251. Causes. In considering the causes which produce association, it is
necessary to call in evidence the primary facts of the process in concrete
examples of this principle. These facts are so bound up in the nature of
vegetal organisms that they are the veriest axioms. Reproduction gives rise
immediately to potential, and ultimately, in the great majority of cases, to
actual association. The degree and permanence of the association are then
determined by the immobility of the individuals as expressed in terms of
attachment to each other or to the stratum, such as sheath, thallus, haustoria,
holdfasts, rhizoids, roots, etc. The range of immobility is very great. In
terrestrial plants, mobility is confined almost entirely to the period when the
individual lies dormant in the seed, spore, or propagative part, which is
alone mobile. In aquatic spermatophytes, the same is true of all attached
forms, while free floating plants such as Lemna are mobile in a high degree,
especially during the vegetative period. Among the algae and hydrophilous
fungi, attached forms are mobile only in the spore or propagative condition,
while the motile forms of the plancton typify the extreme development of
mobility. The immediate result of reproduction in an immobile species is to
produce association of like individuals, while in the case of a mobile
species reproduction may or may not lead immediately to association. We
may lay down the general principle that immobility tends to maintain the
association of the individuals of the same generation, i. e., the association of
like forms, while mobility tends to separate the similar individuals of one
generation and to bring unlike forms together. With the mobile algae,
separation of the members of each generation is the rule, unless the
individuals come to be associated in a thallus, or are grouped in contact
with the substratum. Flowering plants that are relatively immobile,
especially in the seed state, drop their seeds beneath and about the parent
plants, and in consequence dense association of the new plants is the rule.
Page 265
In very many cases, however, this primitive tendency is largely or
completely negatived by the presence of special dissemination contrivances,
which are nearly, if not quite, as effective for many terrestrial plants as the
free floating habit is for algae. From this point, the whole question of
mobility belongs to migration, just as the adjustment between the parent
plants and their offspring, or between plants established and the mobile
plants to be established, belongs to competition.
If association were determined by reproduction and immobility alone, it
would exhibit areas dissimilar in the mass of individuals, as well as areas
dissimilar in the kinds of individuals. Some areas would be occupied by
plants of a single species, others by plants of several or many species. This
tendency of association to show differences is, however, greatly emphasized
by the fact that vegetation is fundamentally attached to and dependent upon
a surface that exhibits the most extreme physical differences. For this
reason, new differences in association appear, due not only to the
morphological differentiation of vegetation forms, but also to the changes in
the degree and manner of association produced directly by the different
habitats. Association might then be defined as a grouping together of plant
individuals, of parents and progeny, which is initiated by reproduction and
immobility, and determined by environment. It is a resultant of differences
and similarities. In consequence, association in its largest expression,
vegetation, is essentially heterogeneous, while in those areas which possess
physical or biological definiteness, habitats and vegetation centers, it is
relatively homogeneous. This fundamental peculiarity has given us the
concept of the formation, an area of vegetation, or a particular association,
which is homogeneous within itself, and at the same time essentially
different from contiguous areas, though falling into a phylogenetic series
with some and a biological series with others. From its nature, the plant
formation is to be considered the logical unit of vegetation, though it is not,
of course, the simplest example of association.
252. Aggregation. As indicated under the causes of association, the
process by which groups of individuals are formed depends entirely upon
reproduction and migration. In short, aggregation is merely a corollary of
movement. The simplest example of this process occurs in forms like
Gloeocapsa, Tetraspora, and others, where the plants resulting from fission
are held together by means of a sheath. Though called a colony, such a
group of individuals is a family in the ordinary sense. Practically the same
completely negatived by the presence of special dissemination contrivances,
which are nearly, if not quite, as effective for many terrestrial plants as the
free floating habit is for algae. From this point, the whole question of
mobility belongs to migration, just as the adjustment between the parent
plants and their offspring, or between plants established and the mobile
plants to be established, belongs to competition.
If association were determined by reproduction and immobility alone, it
would exhibit areas dissimilar in the mass of individuals, as well as areas
dissimilar in the kinds of individuals. Some areas would be occupied by
plants of a single species, others by plants of several or many species. This
tendency of association to show differences is, however, greatly emphasized
by the fact that vegetation is fundamentally attached to and dependent upon
a surface that exhibits the most extreme physical differences. For this
reason, new differences in association appear, due not only to the
morphological differentiation of vegetation forms, but also to the changes in
the degree and manner of association produced directly by the different
habitats. Association might then be defined as a grouping together of plant
individuals, of parents and progeny, which is initiated by reproduction and
immobility, and determined by environment. It is a resultant of differences
and similarities. In consequence, association in its largest expression,
vegetation, is essentially heterogeneous, while in those areas which possess
physical or biological definiteness, habitats and vegetation centers, it is
relatively homogeneous. This fundamental peculiarity has given us the
concept of the formation, an area of vegetation, or a particular association,
which is homogeneous within itself, and at the same time essentially
different from contiguous areas, though falling into a phylogenetic series
with some and a biological series with others. From its nature, the plant
formation is to be considered the logical unit of vegetation, though it is not,
of course, the simplest example of association.
252. Aggregation. As indicated under the causes of association, the
process by which groups of individuals are formed depends entirely upon
reproduction and migration. In short, aggregation is merely a corollary of
movement. The simplest example of this process occurs in forms like
Gloeocapsa, Tetraspora, and others, where the plants resulting from fission
are held together by means of a sheath. Though called a colony, such a
group of individuals is a family in the ordinary sense. Practically the same
Page 266
grouping results in the case of terrestrial plants, especially spermatophytes,
when the seeds of a plant mature and fall to the ground about it. The
relation in both instances is essentially that of parent and offspring,
although the parent soon disappears in the case of annuals, while among the
algae its existence is regularly terminated by fission. The size and the
density of the family group are determined by the number of seeds
produced, and by their mobility. These are further affected by the height and
branching of the plant, and by the position of the seeds upon it. The
disseminules of immobile species fall directly beneath the parent, and the
resulting group is both uniform and definite. A similar arrangement is
caused likewise by offshoots. An increase in mobility brings about a
decrease of aggregation, since the disseminules are carried away from the
parent plant. Perfectly mobile forms rarely produce family groups for this
reason. It is evident, however, that mobile perennials sometimes arrange
themselves in similar fashion in consequence of propagation by
underground parts. Consequently, it is possible to state the law of single
aggregation, viz., that immobility promotes the grouping of parent and
offspring, and mobility hinders it.
If all species were immobile, the family group would be characteristic of
vegetation. Since the great majority are more or less mobile, aggregates of
this sort are the exception rather than the rule. Mobility not only decreases
the number of offspring in the family group, but it also spreads
disseminules broadcast to enter dissimilar groups. It leads directly to mixed
aggregation, by which individuals of one or more species invade the family
group. Once established, the newcomers tend also to produce simple
groups, thus causing an arrangement corresponding essentially to a
community. Such collections of family groups are extremely variable in size
and definition. This arises in part from the nature of simple aggregation, and
in part from the varying mobility of different species. Mobility alone often
produces similar communities by bringing together the disseminules of
different plants, each of which then becomes the center of a mixed group. In
the case of permobile species, several disseminules of each may be brought
together. The resulting area, though larger, is practically the same. At
present, it is difficult to formulate the law for this method of grouping. It
may be stated provisionally as follows: mixed aggregation is the direct
result of mobility, and the greater the mobility the more heterogeneous the
mixture.
when the seeds of a plant mature and fall to the ground about it. The
relation in both instances is essentially that of parent and offspring,
although the parent soon disappears in the case of annuals, while among the
algae its existence is regularly terminated by fission. The size and the
density of the family group are determined by the number of seeds
produced, and by their mobility. These are further affected by the height and
branching of the plant, and by the position of the seeds upon it. The
disseminules of immobile species fall directly beneath the parent, and the
resulting group is both uniform and definite. A similar arrangement is
caused likewise by offshoots. An increase in mobility brings about a
decrease of aggregation, since the disseminules are carried away from the
parent plant. Perfectly mobile forms rarely produce family groups for this
reason. It is evident, however, that mobile perennials sometimes arrange
themselves in similar fashion in consequence of propagation by
underground parts. Consequently, it is possible to state the law of single
aggregation, viz., that immobility promotes the grouping of parent and
offspring, and mobility hinders it.
If all species were immobile, the family group would be characteristic of
vegetation. Since the great majority are more or less mobile, aggregates of
this sort are the exception rather than the rule. Mobility not only decreases
the number of offspring in the family group, but it also spreads
disseminules broadcast to enter dissimilar groups. It leads directly to mixed
aggregation, by which individuals of one or more species invade the family
group. Once established, the newcomers tend also to produce simple
groups, thus causing an arrangement corresponding essentially to a
community. Such collections of family groups are extremely variable in size
and definition. This arises in part from the nature of simple aggregation, and
in part from the varying mobility of different species. Mobility alone often
produces similar communities by bringing together the disseminules of
different plants, each of which then becomes the center of a mixed group. In
the case of permobile species, several disseminules of each may be brought
together. The resulting area, though larger, is practically the same. At
present, it is difficult to formulate the law for this method of grouping. It
may be stated provisionally as follows: mixed aggregation is the direct
result of mobility, and the greater the mobility the more heterogeneous the
mixture.
Page 267
The constitution of all the major areas of a formation is to be explained
upon the basis of aggregation by the two methods described. The relative
importance of family groups and communities differs for every formation,
and the exact procedure in each can be obtained only by the detailed study
of quadrats. The problem is further complicated by competition and
reaction, particularly in closed vegetation. For this reason, aggregation can
be studied most satisfactorily in a new or denuded area, where these
processes are not yet in evidence.
Kinds of Association
253. Categories. In the analysis of association, it must be kept clearly in
mind that the concrete examples from which all generalizations must be
drawn are often in very different stages of development, and are of
correspondingly different ages. For this reason it has seemed best to
consider the primary relations of association in general in this place, leaving
the treatment of the effects of invasion, succession, alternation, and
zonation to be taken up under these topics.
Various categories of association may be distinguished, according to the
dominant physical factor concerned or the point of view taken. These will
fall into two series, as we consider the relation of plant to plant with
reference to some object or characteristic, or the grouping of plants together
in response to some dominant factor. In the first series may be placed
association with reference to substratum, to the ground (occupation), and to
invasion; in the second belong light and water-content association. It should
be noted that these are all actual associations in nature, and not concepts
such as the vegetation form, within which plants from widely different
associations may be classified. Naturally, it does not follow that it is not
logical or valuable to group together those plants, such as hydrophytes,
sciophytes, hysterophytes, etc., which have a common relation to some
factor, but belong to different formations.
254. Stratum association. Plants manifest independent or dependent
association with reference to the stratum to which they are attached and
from which they derive food or support. Independent association is
exhibited by those holophytic species of a formation which are entirely
independent of each other with respect to mechanical support or nutrition. It
upon the basis of aggregation by the two methods described. The relative
importance of family groups and communities differs for every formation,
and the exact procedure in each can be obtained only by the detailed study
of quadrats. The problem is further complicated by competition and
reaction, particularly in closed vegetation. For this reason, aggregation can
be studied most satisfactorily in a new or denuded area, where these
processes are not yet in evidence.
Kinds of Association
253. Categories. In the analysis of association, it must be kept clearly in
mind that the concrete examples from which all generalizations must be
drawn are often in very different stages of development, and are of
correspondingly different ages. For this reason it has seemed best to
consider the primary relations of association in general in this place, leaving
the treatment of the effects of invasion, succession, alternation, and
zonation to be taken up under these topics.
Various categories of association may be distinguished, according to the
dominant physical factor concerned or the point of view taken. These will
fall into two series, as we consider the relation of plant to plant with
reference to some object or characteristic, or the grouping of plants together
in response to some dominant factor. In the first series may be placed
association with reference to substratum, to the ground (occupation), and to
invasion; in the second belong light and water-content association. It should
be noted that these are all actual associations in nature, and not concepts
such as the vegetation form, within which plants from widely different
associations may be classified. Naturally, it does not follow that it is not
logical or valuable to group together those plants, such as hydrophytes,
sciophytes, hysterophytes, etc., which have a common relation to some
factor, but belong to different formations.
254. Stratum association. Plants manifest independent or dependent
association with reference to the stratum to which they are attached and
from which they derive food or support. Independent association is
exhibited by those holophytic species of a formation which are entirely
independent of each other with respect to mechanical support or nutrition. It
Page 268
is characteristic of the greater number of the constituent species of
formations. Dependent association is manifested in the relation between
host and parasite, stratum and epiphyte, support and liane. Warming[24] has
distinguished six kinds of associations: parasitism, helotism, mutualism,
epiphytism, lianism, and commensalism. Commensalism corresponds to the
primary principle of association which has given rise to vegetation.
Homogeneous commensalism is the term applied to social exclusive plants,
in which the patch is composed of a single species. Such association is
extremely rare in nature, and if the most minute forms be considered,
probably never occurs. On the other hand, heterogeneous commensalism, in
which individuals of more than one species are present, is everywhere
typical of vegetation. Warming regards saprophytism merely as a
specialized kind of parasitism, an opinion that may well be defended.
Helotism, however, is also a mere modification of parasitism, if it is not
indeed parasitism pure and simple. Mutualism is an altogether vague
concept, including parasites, epiphytes, and endophytes of doubtful
physiological relation. Pound and Clements[25] treated lianes, parasites, and
saprophytes as vegetation forms, relating herbaceous creepers and twiners
to the lianes, and dividing the fungi and lichens into nine groups. Whatever
the value of these divisions may be from the standpoint of vegetation forms,
they represent the same relation between plant and nutritive stratum, and
with respect to association should be merged in one group. Schimper[26] was
the first to perceive the essential similarity of all such groups from the
standpoint of association. He terms these plant societies
(Genossenschaften), retaining the four groups already established, lianae,
epiphyta, saprophyta, and parasiticae. It is evident that dependent
association comprises extremely divergent forms, from the slightly clinging
herb, such as Galium, to the most intense parasite. The distinction, however,
is a clear one, if restricted to that relation between plants in which one acts
as a mechanical support or stratum or as a nutritive host for the other.
255. Ground association. The first division of formations into open and
closed was made by Engler and Drude.[27] Open formations were defined as
those having incomplete stability and heterogeneous composition, while
closed formations have a more definite uniform stamp. What is true of
formations is equally true of vegetation, so that association may be regarded
as open or closed with reference to the density and thoroughness with
which the plants occupy the ground. In open association, the ground is
formations. Dependent association is manifested in the relation between
host and parasite, stratum and epiphyte, support and liane. Warming[24] has
distinguished six kinds of associations: parasitism, helotism, mutualism,
epiphytism, lianism, and commensalism. Commensalism corresponds to the
primary principle of association which has given rise to vegetation.
Homogeneous commensalism is the term applied to social exclusive plants,
in which the patch is composed of a single species. Such association is
extremely rare in nature, and if the most minute forms be considered,
probably never occurs. On the other hand, heterogeneous commensalism, in
which individuals of more than one species are present, is everywhere
typical of vegetation. Warming regards saprophytism merely as a
specialized kind of parasitism, an opinion that may well be defended.
Helotism, however, is also a mere modification of parasitism, if it is not
indeed parasitism pure and simple. Mutualism is an altogether vague
concept, including parasites, epiphytes, and endophytes of doubtful
physiological relation. Pound and Clements[25] treated lianes, parasites, and
saprophytes as vegetation forms, relating herbaceous creepers and twiners
to the lianes, and dividing the fungi and lichens into nine groups. Whatever
the value of these divisions may be from the standpoint of vegetation forms,
they represent the same relation between plant and nutritive stratum, and
with respect to association should be merged in one group. Schimper[26] was
the first to perceive the essential similarity of all such groups from the
standpoint of association. He terms these plant societies
(Genossenschaften), retaining the four groups already established, lianae,
epiphyta, saprophyta, and parasiticae. It is evident that dependent
association comprises extremely divergent forms, from the slightly clinging
herb, such as Galium, to the most intense parasite. The distinction, however,
is a clear one, if restricted to that relation between plants in which one acts
as a mechanical support or stratum or as a nutritive host for the other.
255. Ground association. The first division of formations into open and
closed was made by Engler and Drude.[27] Open formations were defined as
those having incomplete stability and heterogeneous composition, while
closed formations have a more definite uniform stamp. What is true of
formations is equally true of vegetation, so that association may be regarded
as open or closed with reference to the density and thoroughness with
which the plants occupy the ground. In open association, the ground is
Page 269
slightly or partially occupied, readily permitting the entrance of new plants
without the displacement of those already present. Such an arrangement is
characteristic of the early stages of a formation, or of a succession of
formations. It produces unstable open formations, which arise, usually after
denudation, in sand-hills, blowouts, gravel slides, dunes, flood plains,
burned areas, etc. In closed association, occupation of the ground is
complete, and the invasion of new species can occur only through
displacement. Closed association results in stable, closed formations, such
as forest, thicket, meadow, and prairie. As open association characterizes
the early stages of a succession of formations, so closed association is
peculiar to the later or last stages of all such successions. In short, open
formations represent certain phases of the development of vegetation, while
closed formations correspond to the relatively final structural conditions. It
is a fundamental principle of association that every succession from
denudation, or from newly formed soils, begins with open formations and
ends with a closed formation. The causes leading up to open and closed
association are intimately connected with development, and hence are
considered under invasion and succession.
256. Species guild association. Drude has distinguished a kind of
association peculiar to invasion, in which there is a successive or
concomitant movement of certain species of a formation into another
formation or region, resulting in species guilds (Artengenossenschaften).
The association in this case is largely one of community of origin or area,
and of concomitant migration. It is especially characteristic of areas
adjacent to formational and regional limits. Fundamentally, it is merely the
grouping of plants which are invading at the same time, and consequently it
differs only in degree from what occurs in every invasion where more than
a single individual is concerned. Accordingly, this type of association has
little more than historical interest. This must not be construed to mean that
it does not occur, but that it differs in no essential from the ordinary
grouping of invaders.
257. Light association. The constituent species of formations show two
fundamentally different groupings with respect to light. In the one case, the
individuals are on the same level, or nearly so, in such a way that each has
direct access to sunlight. Such an arrangement is characteristic of most
grassland and herbaceous formations. In the case of desert formations, there
is often considerable difference in the height of the plants, but the distance
without the displacement of those already present. Such an arrangement is
characteristic of the early stages of a formation, or of a succession of
formations. It produces unstable open formations, which arise, usually after
denudation, in sand-hills, blowouts, gravel slides, dunes, flood plains,
burned areas, etc. In closed association, occupation of the ground is
complete, and the invasion of new species can occur only through
displacement. Closed association results in stable, closed formations, such
as forest, thicket, meadow, and prairie. As open association characterizes
the early stages of a succession of formations, so closed association is
peculiar to the later or last stages of all such successions. In short, open
formations represent certain phases of the development of vegetation, while
closed formations correspond to the relatively final structural conditions. It
is a fundamental principle of association that every succession from
denudation, or from newly formed soils, begins with open formations and
ends with a closed formation. The causes leading up to open and closed
association are intimately connected with development, and hence are
considered under invasion and succession.
256. Species guild association. Drude has distinguished a kind of
association peculiar to invasion, in which there is a successive or
concomitant movement of certain species of a formation into another
formation or region, resulting in species guilds (Artengenossenschaften).
The association in this case is largely one of community of origin or area,
and of concomitant migration. It is especially characteristic of areas
adjacent to formational and regional limits. Fundamentally, it is merely the
grouping of plants which are invading at the same time, and consequently it
differs only in degree from what occurs in every invasion where more than
a single individual is concerned. Accordingly, this type of association has
little more than historical interest. This must not be construed to mean that
it does not occur, but that it differs in no essential from the ordinary
grouping of invaders.
257. Light association. The constituent species of formations show two
fundamentally different groupings with respect to light. In the one case, the
individuals are on the same level, or nearly so, in such a way that each has
direct access to sunlight. Such an arrangement is characteristic of most
grassland and herbaceous formations. In the case of desert formations, there
is often considerable difference in the height of the plants, but the distance
Page 270
between them is so great as to admit of direct illumination of all. This
arrangement may be termed coordinate association. In forests, thickets, and
many herbaceous wastes, the height and density of certain species enable
them to dominate the formation. In a dense forest, the trees receive
practically all the light incident upon the formation, and the shrubs, herbs,
fungi, and algae of lower habit and inferior position must adapt themselves
to the diffuse light which passes through or between the leaves. The same is
equally true of dense thickets and wastes, except that the vertical distance is
less, and the diffuseness of the light is correspondingly modified. In these
formations, the dominant trees, shrubs, or herbs, the facies, constitute a
primary or superior layer. The degree of subordinate association, as a result
of which inferior layers will arise, is entirely determined by the density of
the facies. In open woodlands, which are really mixed formations of
woodland and grassland, the intervals, and usually the spaces beneath the
trees also, are covered with poophytes, showing an absence of
subordination due to light. This is the prevailing condition in the pine
formation (Pinus ponderosa-xerohylium) of the ridges and foot-hills of
western Nebraska. When, however, the trees stand sufficiently close that
their shadows meet or overlap throughout the day, the increasing
diffuseness begins to cause modification and rearrangement of the
individuals. By photometric methods, the light in a forest is found to be
least diffuse just below the facies, while the diffuseness increases markedly
in passing to the ground. The taller, stronger individuals are consequently in
a position to assimilate more vigorously, and to become still taller and
stronger as a result. Just as these have taken up a position inferior to that of
the facies, so the shorter or weaker species must come to occupy a still
more subordinate position. This results, not only because the light is
primarily weaker nearer the ground, but also because the taller plants
interpose as a second screen. The complete working out of this arrangement
with reference to light produces typical subordinate association, which finds
its characteristic expression in the layering of forests and thickets. Layers
tend to appear as soon as open woodland or thicket begins to pass into
denser conditions, and up to a certain point, at which they disappear, they
become the more numerous and the more marked, the denser the forest.
In the Otowanie woods near Lincoln (Quercus-Hicoria-hylium), layering
usually begins at a light value of .1 (1 = normal sunshine in the open).
Thornber[28] has found the same value to obtain in the thickets of the
arrangement may be termed coordinate association. In forests, thickets, and
many herbaceous wastes, the height and density of certain species enable
them to dominate the formation. In a dense forest, the trees receive
practically all the light incident upon the formation, and the shrubs, herbs,
fungi, and algae of lower habit and inferior position must adapt themselves
to the diffuse light which passes through or between the leaves. The same is
equally true of dense thickets and wastes, except that the vertical distance is
less, and the diffuseness of the light is correspondingly modified. In these
formations, the dominant trees, shrubs, or herbs, the facies, constitute a
primary or superior layer. The degree of subordinate association, as a result
of which inferior layers will arise, is entirely determined by the density of
the facies. In open woodlands, which are really mixed formations of
woodland and grassland, the intervals, and usually the spaces beneath the
trees also, are covered with poophytes, showing an absence of
subordination due to light. This is the prevailing condition in the pine
formation (Pinus ponderosa-xerohylium) of the ridges and foot-hills of
western Nebraska. When, however, the trees stand sufficiently close that
their shadows meet or overlap throughout the day, the increasing
diffuseness begins to cause modification and rearrangement of the
individuals. By photometric methods, the light in a forest is found to be
least diffuse just below the facies, while the diffuseness increases markedly
in passing to the ground. The taller, stronger individuals are consequently in
a position to assimilate more vigorously, and to become still taller and
stronger as a result. Just as these have taken up a position inferior to that of
the facies, so the shorter or weaker species must come to occupy a still
more subordinate position. This results, not only because the light is
primarily weaker nearer the ground, but also because the taller plants
interpose as a second screen. The complete working out of this arrangement
with reference to light produces typical subordinate association, which finds
its characteristic expression in the layering of forests and thickets. Layers
tend to appear as soon as open woodland or thicket begins to pass into
denser conditions, and up to a certain point, at which they disappear, they
become the more numerous and the more marked, the denser the forest.
In the Otowanie woods near Lincoln (Quercus-Hicoria-hylium), layering
usually begins at a light value of .1 (1 = normal sunshine in the open).
Thornber[28] has found the same value to obtain in the thickets of the
Page 271
Missouri bluffs. In these, again, layers disappear at a value of .005, the
extreme diffuseness making assimilation impossible except for occasional
mosses and algae. A number of herbaceous plants are present in the spring,
but these are all prevernal or vernal bloomers, which are safely past
flowering before shade conditions become extreme. In the Fraxinus-
Catalpa-alsium, all inferior holophytic vegetation disappears between the
light value of .004 and that of .003. The spruce-pine formation (Picea-
Pinus-hylium) of the Rocky mountains, with a light value of .01, usually
contains but a few scattered herbs, mostly evergreen; in some cases there
are no subordinate plants other than mosses and hysterophytes. The lodge-
pole pine formation (Pinus murrayana-hylium), with light values often less
than .005, is nearly or quite destitute of all but hysterophytic undergrowth.
Such extremely dense formations are examples of coordinate association
merely, since the formation is reduced to a single superior layer, in which
the individuals of the facies bear the same spatial relation to incident light.
In layered formations, in addition to the subordinate relation of other
species to the facies, there is, of course, a kind of coordinate association
manifested in each layer.
258. Water-content association. Schouw[29] was the first to give definite
expression to the value of the water-content of the soil for the grouping of
plants. He established four groups: (1) water plants, (2) swamp plants, (3)
plants of moist meadows, (4) plants of dry soils. The first he termed
hydrophytes, introducing the term halophytes to include all saline plants.
Thurmann[30] recognized the fundamental influence of water-content upon
association, and further perceived that the amount of water present was
determined primarily by the physical nature of the soil. He distinguished
plants which grow in soils that retain water as hygrophilous, and those
found upon soils that lose water readily as xerophilous. Those which
seemed to grow indifferently upon either were termed ubiquitous. The latter
correspond in some measure to mesophytes, but they are really plants
possessing a considerable range of adaptability, and do not properly
constitute a natural group. Warming[31] proposed the term mesophytes to
include all the plants intermediate between hydrophytes and xerophytes. He
recognized the paramount value of water-content association as the basis of
ecology, and upon this made a logical and systematic treatise out of the
scattered results of many workers. Schimper[32] placed the study of
vegetation upon a new basis by drawing a distinction between physical and
extreme diffuseness making assimilation impossible except for occasional
mosses and algae. A number of herbaceous plants are present in the spring,
but these are all prevernal or vernal bloomers, which are safely past
flowering before shade conditions become extreme. In the Fraxinus-
Catalpa-alsium, all inferior holophytic vegetation disappears between the
light value of .004 and that of .003. The spruce-pine formation (Picea-
Pinus-hylium) of the Rocky mountains, with a light value of .01, usually
contains but a few scattered herbs, mostly evergreen; in some cases there
are no subordinate plants other than mosses and hysterophytes. The lodge-
pole pine formation (Pinus murrayana-hylium), with light values often less
than .005, is nearly or quite destitute of all but hysterophytic undergrowth.
Such extremely dense formations are examples of coordinate association
merely, since the formation is reduced to a single superior layer, in which
the individuals of the facies bear the same spatial relation to incident light.
In layered formations, in addition to the subordinate relation of other
species to the facies, there is, of course, a kind of coordinate association
manifested in each layer.
258. Water-content association. Schouw[29] was the first to give definite
expression to the value of the water-content of the soil for the grouping of
plants. He established four groups: (1) water plants, (2) swamp plants, (3)
plants of moist meadows, (4) plants of dry soils. The first he termed
hydrophytes, introducing the term halophytes to include all saline plants.
Thurmann[30] recognized the fundamental influence of water-content upon
association, and further perceived that the amount of water present was
determined primarily by the physical nature of the soil. He distinguished
plants which grow in soils that retain water as hygrophilous, and those
found upon soils that lose water readily as xerophilous. Those which
seemed to grow indifferently upon either were termed ubiquitous. The latter
correspond in some measure to mesophytes, but they are really plants
possessing a considerable range of adaptability, and do not properly
constitute a natural group. Warming[31] proposed the term mesophytes to
include all the plants intermediate between hydrophytes and xerophytes. He
recognized the paramount value of water-content association as the basis of
ecology, and upon this made a logical and systematic treatise out of the
scattered results of many workers. Schimper[32] placed the study of
vegetation upon a new basis by drawing a distinction between physical and
Page 272
physiological water-content, and by pointing out that the last alone is to be
taken into account in the study of plant life, and hence of plant geography.
Accepting the easily demonstrable fact that an excess of salts in the soil
water, as well as cold, tends greatly to diminish the available water of the
soil, i. e., the chresard, it is at once seen why saline and arctic plants are as
truly xerophytic as those that grow on rocks or in desert sands. An
anomalous case which, however, physical factor records have explained
fully, is presented by many plants growing in alpine gravel slides, strands,
blowouts, sandbars, etc., in which the water-content is considerable, but the
water loss excessive, on account of extreme heat or reduced air pressure.
The effect of these conditions is to produce a plant xerophytic as to its aerial
parts, and mesophytic or even hydrophytic as to subterranean parts. Such
plants may, from their twofold nature, be termed dissophytes; they are
especially characteristic of dysgeogenous soils in alpine regions where
transpiration reaches a maximum, but are doubtless to be found in all gravel
and sand habitats with high water-content. With these corrections, the
concept of water-content association, which owes much to both Warming
and Schimper, but is largely to be credited to Thurmann, becomes
completely and fundamentally applicable to all vegetation.
Up to the present time, the general character of the habitat, together with
the gross appearance of the plant itself, has been thought sufficient to
determine the proper position of a plant or a formation in the water-content
classification. Such a method is adequate, however, only for plants and
formations which bear a distinct impress. For an accurate classification into
the three categories, hydrophytes, mesophytes, and xerophytes, it is
necessary to make exact determinations of the normal holard and chresard
of the habitat, and to supplement this, in some degree at least, by
histological studies. Except in the case of saline, acid, and frozen soils, the
holard alone will be a fairly accurate index, especially in habitats of similar
soil composition. For an exact and comprehensive classification, however,
and particularly in comparative work, the chresard must constitute the sole
criterion. As the latter has been ascertained for very few formations, and in
Nebraska and Colorado alone, the present characterization of many plants
and formations as hydrophytic, mesophytic, or xerophytic must be regarded
as largely tentative, and the final classification will be possible only after
the thorough quantitative investigation of their habitats.
taken into account in the study of plant life, and hence of plant geography.
Accepting the easily demonstrable fact that an excess of salts in the soil
water, as well as cold, tends greatly to diminish the available water of the
soil, i. e., the chresard, it is at once seen why saline and arctic plants are as
truly xerophytic as those that grow on rocks or in desert sands. An
anomalous case which, however, physical factor records have explained
fully, is presented by many plants growing in alpine gravel slides, strands,
blowouts, sandbars, etc., in which the water-content is considerable, but the
water loss excessive, on account of extreme heat or reduced air pressure.
The effect of these conditions is to produce a plant xerophytic as to its aerial
parts, and mesophytic or even hydrophytic as to subterranean parts. Such
plants may, from their twofold nature, be termed dissophytes; they are
especially characteristic of dysgeogenous soils in alpine regions where
transpiration reaches a maximum, but are doubtless to be found in all gravel
and sand habitats with high water-content. With these corrections, the
concept of water-content association, which owes much to both Warming
and Schimper, but is largely to be credited to Thurmann, becomes
completely and fundamentally applicable to all vegetation.
Up to the present time, the general character of the habitat, together with
the gross appearance of the plant itself, has been thought sufficient to
determine the proper position of a plant or a formation in the water-content
classification. Such a method is adequate, however, only for plants and
formations which bear a distinct impress. For an accurate classification into
the three categories, hydrophytes, mesophytes, and xerophytes, it is
necessary to make exact determinations of the normal holard and chresard
of the habitat, and to supplement this, in some degree at least, by
histological studies. Except in the case of saline, acid, and frozen soils, the
holard alone will be a fairly accurate index, especially in habitats of similar
soil composition. For an exact and comprehensive classification, however,
and particularly in comparative work, the chresard must constitute the sole
criterion. As the latter has been ascertained for very few formations, and in
Nebraska and Colorado alone, the present characterization of many plants
and formations as hydrophytic, mesophytic, or xerophytic must be regarded
as largely tentative, and the final classification will be possible only after
the thorough quantitative investigation of their habitats.
Page 273
The water-content groups, hydrophytia, mesophytia, and xerophytia,
include all formations found upon the globe. The exactness with which this
classification applies to vegetation is made somewhat more evident by
dividing mesophytia into forest and grassland. This is based primarily upon
light association, but it also reflects water-content differences in a large
degree. The groups thus constituted represent the fundamental zonation of
the vegetative covering with respect to water-content. Ocean, forest,
grassland, and desert correspond exactly to hydrophytia, hylophytia,
poophytia, and xerophytia. The difference is merely one of terminology: the
first series takes into account the physiognomy of the vegetation itself,
while the other emphasizes the causative factors.
include all formations found upon the globe. The exactness with which this
classification applies to vegetation is made somewhat more evident by
dividing mesophytia into forest and grassland. This is based primarily upon
light association, but it also reflects water-content differences in a large
degree. The groups thus constituted represent the fundamental zonation of
the vegetative covering with respect to water-content. Ocean, forest,
grassland, and desert correspond exactly to hydrophytia, hylophytia,
poophytia, and xerophytia. The difference is merely one of terminology: the
first series takes into account the physiognomy of the vegetation itself,
while the other emphasizes the causative factors.
Page 274
THE DEVELOPMENT OF THE FORMATION
259. A strict account of development should trace the results of the
various activities of vegetation in their proper sequence. This is
aggregation, migration, ecesis, reaction, and competition. These functions
are so intimately and often so inextricably associated that it is hardly
feasible to discuss development by treating each one separately. In
consequence, the two fundamental phenomena, invasion and succession,
which they produce, are taken as the basis of the discussion. These,
moreover, are different only in degree; succession is merely complete,
periodic invasion. Nevertheless, the subject gains in clearness by a separate
treatment of each.
259. A strict account of development should trace the results of the
various activities of vegetation in their proper sequence. This is
aggregation, migration, ecesis, reaction, and competition. These functions
are so intimately and often so inextricably associated that it is hardly
feasible to discuss development by treating each one separately. In
consequence, the two fundamental phenomena, invasion and succession,
which they produce, are taken as the basis of the discussion. These,
moreover, are different only in degree; succession is merely complete,
periodic invasion. Nevertheless, the subject gains in clearness by a separate
treatment of each.
Page 275
INVASION
260. By invasion is understood the movement of plants from an area of a
certain character into one of a different character, and their colonization in
the latter. This movement may concern an individual, a species, or a group
of species. From the nature of invasion, which contains the double idea of
going into and taking possession of, it usually operates between contiguous
formations, but it also takes place between formational zones and patches.
More rarely and less noticeably, there may be invasion into a remote
vegetation, as a result of long carriage by wind, water, birds, railroads, or
vessels. Movement or migration, however, represents but one of the two
ideas involved in invasion. Migration merely carries the spore, seed, or
propagule into the area to be invaded. In ecesis, the spores or seeds
germinate and grow, after more or less adjustment, and in case the latter
becomes sufficiently complete, the new plants reproduce and finally
become established. With all terrestrial plants, invasion is possible only
when migration is followed by ecesis, because of the inherent differences of
formations or of areas of the same formation. In the case of surface floating
forms, such as Lemnaceae, and of the plancton, ecesis is of much less
importance, on account of the uniformity of the medium and the lack of
attachment, and migration is often practically synonymous with invasion.
MIGRATION
261. Migration has been sometimes used loosely as a synonym for
invasion, but it is here employed in its proper sense of removal or departure,
i. e., movement, and is contrasted with ecesis, the making of a home, the
two ideas being combined in invasion, which is a moving into and a taking
possession of. An analysis of migration reveals the presence of four factors,
mobility, agency, proximity, and topography. Not all of these are present in
every instance of migration, as for example in the simple elongation of a
rootstalk, but in the great majority of cases each plays its proper part.
Mobility represents the inherent capacity of a plant for migration, and in its
highest expression, motility, is in itself productive of movement. As a
260. By invasion is understood the movement of plants from an area of a
certain character into one of a different character, and their colonization in
the latter. This movement may concern an individual, a species, or a group
of species. From the nature of invasion, which contains the double idea of
going into and taking possession of, it usually operates between contiguous
formations, but it also takes place between formational zones and patches.
More rarely and less noticeably, there may be invasion into a remote
vegetation, as a result of long carriage by wind, water, birds, railroads, or
vessels. Movement or migration, however, represents but one of the two
ideas involved in invasion. Migration merely carries the spore, seed, or
propagule into the area to be invaded. In ecesis, the spores or seeds
germinate and grow, after more or less adjustment, and in case the latter
becomes sufficiently complete, the new plants reproduce and finally
become established. With all terrestrial plants, invasion is possible only
when migration is followed by ecesis, because of the inherent differences of
formations or of areas of the same formation. In the case of surface floating
forms, such as Lemnaceae, and of the plancton, ecesis is of much less
importance, on account of the uniformity of the medium and the lack of
attachment, and migration is often practically synonymous with invasion.
MIGRATION
261. Migration has been sometimes used loosely as a synonym for
invasion, but it is here employed in its proper sense of removal or departure,
i. e., movement, and is contrasted with ecesis, the making of a home, the
two ideas being combined in invasion, which is a moving into and a taking
possession of. An analysis of migration reveals the presence of four factors,
mobility, agency, proximity, and topography. Not all of these are present in
every instance of migration, as for example in the simple elongation of a
rootstalk, but in the great majority of cases each plays its proper part.
Mobility represents the inherent capacity of a plant for migration, and in its
highest expression, motility, is in itself productive of movement. As a
Page 276
general rule, however, modifications for securing mobility are ineffective in
the absence of proper agents, and the effective operation of the two will be
profoundly influenced by distance and topography.
262. Mobility denotes potentiality of migration as represented by
modifications for this purpose. It corresponds, in a sense, to dissemination,
though seed production also enters into it. Its most perfect expression is
found in those plants which are themselves motile, Bacteriaceae,
Oscillatoria, Volvocaceae, and Bacillariaceae, or possess motile
propagules, such as most Phycophyta. On the other hand, it is entirely
undeveloped in many plants with heavy unspecialized seeds and fruits.
Between these two extremes lie by far the greater number of plants,
exhibiting the most various degrees of mobility, from the motile though
almost immobile offshoots of many Liliaceae to the immotile but very
mobile spores of fungi. It is thus seen that motility plays a relatively small
part in migration, being practically absent in terrestrial forms, and that it
bears a very uncertain relation to mobility. In analyzing the latter,
contrivances for dissemination are seen to determine primarily the degree of
mobility, while the number of seeds produced will have an important effect
in increasing or decreasing it. A third factor of considerable importance is
also involved, namely, position with reference to the distributive agent, but
any exact knowledge of its importance must await systematic experiment
somewhat after the methods of Dingler, but with air-currents, etc., of known
velocity and direction. The time is not distant when by such methods it will
be possible to establish a coefficient of mobility, derived from terms of
position, weight, resistant surface, and trajectory for definite wind velocities
or for particular propulsive mechanisms.
263. Organs for dissemination. Plants exhibit considerable diversity
with reference to the part or organ modified, or at least utilized, for
dissemination. This modification, though usually affecting the particular
product of reproduction, may, in fact, operate on any part of the plant, and
in certain cases upon the entire plant itself. In the majority of plants
characterized by alternation of generations, the same individual may be
disseminated in one generation by a reproductive body, and in the other by
a propagative one, as is the case in the oogones and conidia of
Peronospora, the spores and gemmae of Marchantia, the fruits and runners
of Fragaria, etc. Special modifications have, as a rule, been developed in
direct connection with spores and seeds, and mobility reaches its highest
the absence of proper agents, and the effective operation of the two will be
profoundly influenced by distance and topography.
262. Mobility denotes potentiality of migration as represented by
modifications for this purpose. It corresponds, in a sense, to dissemination,
though seed production also enters into it. Its most perfect expression is
found in those plants which are themselves motile, Bacteriaceae,
Oscillatoria, Volvocaceae, and Bacillariaceae, or possess motile
propagules, such as most Phycophyta. On the other hand, it is entirely
undeveloped in many plants with heavy unspecialized seeds and fruits.
Between these two extremes lie by far the greater number of plants,
exhibiting the most various degrees of mobility, from the motile though
almost immobile offshoots of many Liliaceae to the immotile but very
mobile spores of fungi. It is thus seen that motility plays a relatively small
part in migration, being practically absent in terrestrial forms, and that it
bears a very uncertain relation to mobility. In analyzing the latter,
contrivances for dissemination are seen to determine primarily the degree of
mobility, while the number of seeds produced will have an important effect
in increasing or decreasing it. A third factor of considerable importance is
also involved, namely, position with reference to the distributive agent, but
any exact knowledge of its importance must await systematic experiment
somewhat after the methods of Dingler, but with air-currents, etc., of known
velocity and direction. The time is not distant when by such methods it will
be possible to establish a coefficient of mobility, derived from terms of
position, weight, resistant surface, and trajectory for definite wind velocities
or for particular propulsive mechanisms.
263. Organs for dissemination. Plants exhibit considerable diversity
with reference to the part or organ modified, or at least utilized, for
dissemination. This modification, though usually affecting the particular
product of reproduction, may, in fact, operate on any part of the plant, and
in certain cases upon the entire plant itself. In the majority of plants
characterized by alternation of generations, the same individual may be
disseminated in one generation by a reproductive body, and in the other by
a propagative one, as is the case in the oogones and conidia of
Peronospora, the spores and gemmae of Marchantia, the fruits and runners
of Fragaria, etc. Special modifications have, as a rule, been developed in
direct connection with spores and seeds, and mobility reaches its highest
Page 277
expression in these. It is, on the other hand, greatly restricted in offshoots
and plant bodies, at least in terrestrial forms, though it will now and then
attain a marked development in these, as shown by the rosettes of
Sempervivum and the tumbling plants of Cycloloma. For the sake of
convenience, in analyzing migration, all plants may be arranged in the
following groups with reference to the organ or part distributed.
1. Spore-distributed, sporostrotes. This includes all plants possessing
structures which go by the name of spore, such as the acinetes of Nostoc
and Protococcus, the zoogonidia of Ulothrix, Ectocarpus, etc., the conidia,
ascospores, and basidiospores of fungi, the tetraspores of red seaweeds, and
the gemmae and spores proper of liverworts, mosses, and ferns. These are
almost always without especial contrivances for dissemination, but their
extreme minuteness results in great mobility.
2. Seed-distributed, spermatostrotes. This group comprises all flowering
plants in which the seed is the part modified or at least disseminated. The
mobility of seeds is relatively small, except in the case of minute, winged or
comate seeds.
3. Fruit-distributed, carpostrotes. The modifications of the fruit for
distribution exceed in number and variety all other modifications of this
sort. All achenes, perigynia, utricles, etc., properly belong here.
4. Offshoot-distributed, thallostrotes. To this class are referred those
plants, almost exclusively cormophytes, which produce lateral, branch-like
propagules, such as root-sprouts, rhizomes, runners, stolons, rosettes, etc.
Migration with such plants is extremely slow, but correspondingly effective,
since it is almost invariably followed by ecesis.
5. Plant-distributed, phytostrotes. This group includes all plancton and
surface forms, whether motile or non-motile, and those terrestrial plants in
which the whole plant, or at least the aerial part, is distributed, as in
tumbleweeds and in many grasses.
264. Contrivances for dissemination. Any investigation of migration to
be exact must confine itself to fixed forms. For these the degree of
perfection shown by dissemination contrivances corresponds almost exactly
to the degree of mobility. Because of the difficulty of ascertaining the effect
of ecesis, it is impossible to determine the actual effectiveness in nature of
different modifications, and the best that can be done at present is to regard
mobility, together with the occurrence and forcefulness of distributive
and plant bodies, at least in terrestrial forms, though it will now and then
attain a marked development in these, as shown by the rosettes of
Sempervivum and the tumbling plants of Cycloloma. For the sake of
convenience, in analyzing migration, all plants may be arranged in the
following groups with reference to the organ or part distributed.
1. Spore-distributed, sporostrotes. This includes all plants possessing
structures which go by the name of spore, such as the acinetes of Nostoc
and Protococcus, the zoogonidia of Ulothrix, Ectocarpus, etc., the conidia,
ascospores, and basidiospores of fungi, the tetraspores of red seaweeds, and
the gemmae and spores proper of liverworts, mosses, and ferns. These are
almost always without especial contrivances for dissemination, but their
extreme minuteness results in great mobility.
2. Seed-distributed, spermatostrotes. This group comprises all flowering
plants in which the seed is the part modified or at least disseminated. The
mobility of seeds is relatively small, except in the case of minute, winged or
comate seeds.
3. Fruit-distributed, carpostrotes. The modifications of the fruit for
distribution exceed in number and variety all other modifications of this
sort. All achenes, perigynia, utricles, etc., properly belong here.
4. Offshoot-distributed, thallostrotes. To this class are referred those
plants, almost exclusively cormophytes, which produce lateral, branch-like
propagules, such as root-sprouts, rhizomes, runners, stolons, rosettes, etc.
Migration with such plants is extremely slow, but correspondingly effective,
since it is almost invariably followed by ecesis.
5. Plant-distributed, phytostrotes. This group includes all plancton and
surface forms, whether motile or non-motile, and those terrestrial plants in
which the whole plant, or at least the aerial part, is distributed, as in
tumbleweeds and in many grasses.
264. Contrivances for dissemination. Any investigation of migration to
be exact must confine itself to fixed forms. For these the degree of
perfection shown by dissemination contrivances corresponds almost exactly
to the degree of mobility. Because of the difficulty of ascertaining the effect
of ecesis, it is impossible to determine the actual effectiveness in nature of
different modifications, and the best that can be done at present is to regard
mobility, together with the occurrence and forcefulness of distributive
Page 278
agents, as an approximate measure of migration. The general accuracy of
such a measure will be more or less evident from the following. Of 118
species common to the foot-hill and sand-hill regions of Nebraska, regions
which are sufficiently diverse to indicate that these common species must
have entered either one by migration from the other, 83 exhibit
modifications for dissemination, while 8 others, though without special
contrivances, are readily distributed by water, and 4 more are mobile
because of minuteness of spore or seed. Some degree of mobility is present
in 73 per cent of the species common to these regions, while of the total
number of species in which the mode of migration is evident, viz., 95, 66
per cent are wind-distributed, 20 per cent animal-distributed, and 14 per
cent are water-distributed. It need hardly be noted that this accords fully
with the prevalence and forcefulness of winds in these regions. Of the
species peculiar to the foot-hill region, many are doubtless indigenous,
though a majority have come from the montane regions to the westward.
The number of mobile species is 121, or 60 per cent of the entire number,
while the number of wind-distributed ones is 85, or 70 per cent of those that
are mobile. Among the 25 species found in the widely separated wooded
bluff and foot-hill regions, 2 only, Amorpha nana and Roripa nasturtium,
are relatively immobile, but the minute seeds of the latter, however, are
readily distributed, and the former is altogether infrequent.
The following groups of plants may be distinguished according to the
character of the contrivance by which dissemination is secured:
1. Saccate, saccospores. Here are to be placed a variety of fruits, all of
which agree, however, in having a membranous envelope or an impervious,
air-containing pericarp. In Ostrya, Physalis, Staphylea, the modification is
for wind-distribution, while in Carex, Nymphaea, etc., it is for water-
transport.
2. Winged, pterospores. This group includes all winged, margined, and
flattened fruits and seeds, such as are found in Acer, Betula, Rumex, many
Umbelliferae, Graminaceae, etc.
3. Comate, comospores. To this group belong those fruits and seeds with
long silky hairs, Gossypium, Anemone, Asclepias, etc., and those with
straight capillary hairs or bristles not confined to one end, Typha, Salix, etc.
4. Parachute, petasospores. The highly developed members of this group,
Taraxacum, Lactuca, and other Liguliflorae are connected through Senecio
such a measure will be more or less evident from the following. Of 118
species common to the foot-hill and sand-hill regions of Nebraska, regions
which are sufficiently diverse to indicate that these common species must
have entered either one by migration from the other, 83 exhibit
modifications for dissemination, while 8 others, though without special
contrivances, are readily distributed by water, and 4 more are mobile
because of minuteness of spore or seed. Some degree of mobility is present
in 73 per cent of the species common to these regions, while of the total
number of species in which the mode of migration is evident, viz., 95, 66
per cent are wind-distributed, 20 per cent animal-distributed, and 14 per
cent are water-distributed. It need hardly be noted that this accords fully
with the prevalence and forcefulness of winds in these regions. Of the
species peculiar to the foot-hill region, many are doubtless indigenous,
though a majority have come from the montane regions to the westward.
The number of mobile species is 121, or 60 per cent of the entire number,
while the number of wind-distributed ones is 85, or 70 per cent of those that
are mobile. Among the 25 species found in the widely separated wooded
bluff and foot-hill regions, 2 only, Amorpha nana and Roripa nasturtium,
are relatively immobile, but the minute seeds of the latter, however, are
readily distributed, and the former is altogether infrequent.
The following groups of plants may be distinguished according to the
character of the contrivance by which dissemination is secured:
1. Saccate, saccospores. Here are to be placed a variety of fruits, all of
which agree, however, in having a membranous envelope or an impervious,
air-containing pericarp. In Ostrya, Physalis, Staphylea, the modification is
for wind-distribution, while in Carex, Nymphaea, etc., it is for water-
transport.
2. Winged, pterospores. This group includes all winged, margined, and
flattened fruits and seeds, such as are found in Acer, Betula, Rumex, many
Umbelliferae, Graminaceae, etc.
3. Comate, comospores. To this group belong those fruits and seeds with
long silky hairs, Gossypium, Anemone, Asclepias, etc., and those with
straight capillary hairs or bristles not confined to one end, Typha, Salix, etc.
4. Parachute, petasospores. The highly developed members of this group,
Taraxacum, Lactuca, and other Liguliflorae are connected through Senecio
Page 279
and Eriophorum with the preceding. These represent the highest
development of mobility attained by special modification.
5. Chaffy-pappose, carphospores. In this group are placed those achenes
with a more or less scaly or chaffy pappus with slight mobility, as in
Rudbeckia, Brauneria, Helianthus, etc.
6. Plumed, lophospores. In the fruits of this class, the style is the part
usually modified into a long plumose organ, possessing a high degree of
mobility, as in Pulsatilla, Sieversia, and Clematis.
7. Awned, ascospores. These are almost exclusively grasses, in which the
awns serve for distribution by wind, water, or animals, and even, according
to Kerner, by hygroscopic creeping movements. The mobility in many cases
is great.
8. Spiny, centrospores. This group contains a few representatives which
possess a moderate degree of mobility by attachment, as in Tribulus and
Cenchrus.
9. Hooked, oncospores. The members of this group are extremely
numerous, and the degree of mobility as a rule is very high. All exhibit in
common the development of hooks or barbs, by which they are
disseminated in consequence of attachment, though the number, size, and
disposition of the hooks vary exceedingly.
10. Viscid, gloeospores. In these, the inflorescence is more or less
covered with a viscid substance, as in species of Silene, or the fruit is beset
with glandular hairs, as in Cerastium, Salvia, etc.
11. Fleshy, sarcospores. These are intended for dissemination by
deglutition, largely by birds; the effectiveness of the modification depends
in a large degree upon the resistance of the seed envelope to digestion. The
mobility varies greatly, but the area over which migration may be effected
is large.
12. Nut-fruited, creatospores. This group includes those plants with nut
fruits which are carried away and secreted by animals for food.
13. Flagellate, mastigospores. These are plants with ciliate or flagellate
propagative cells, i. e., zoogonidia, as in Protococcus, Ulothrix,
Oedogonium, Ectocarpus, etc., or with plant bodies similarly motile,
Bacteriaceae and Volvocaceae.
development of mobility attained by special modification.
5. Chaffy-pappose, carphospores. In this group are placed those achenes
with a more or less scaly or chaffy pappus with slight mobility, as in
Rudbeckia, Brauneria, Helianthus, etc.
6. Plumed, lophospores. In the fruits of this class, the style is the part
usually modified into a long plumose organ, possessing a high degree of
mobility, as in Pulsatilla, Sieversia, and Clematis.
7. Awned, ascospores. These are almost exclusively grasses, in which the
awns serve for distribution by wind, water, or animals, and even, according
to Kerner, by hygroscopic creeping movements. The mobility in many cases
is great.
8. Spiny, centrospores. This group contains a few representatives which
possess a moderate degree of mobility by attachment, as in Tribulus and
Cenchrus.
9. Hooked, oncospores. The members of this group are extremely
numerous, and the degree of mobility as a rule is very high. All exhibit in
common the development of hooks or barbs, by which they are
disseminated in consequence of attachment, though the number, size, and
disposition of the hooks vary exceedingly.
10. Viscid, gloeospores. In these, the inflorescence is more or less
covered with a viscid substance, as in species of Silene, or the fruit is beset
with glandular hairs, as in Cerastium, Salvia, etc.
11. Fleshy, sarcospores. These are intended for dissemination by
deglutition, largely by birds; the effectiveness of the modification depends
in a large degree upon the resistance of the seed envelope to digestion. The
mobility varies greatly, but the area over which migration may be effected
is large.
12. Nut-fruited, creatospores. This group includes those plants with nut
fruits which are carried away and secreted by animals for food.
13. Flagellate, mastigospores. These are plants with ciliate or flagellate
propagative cells, i. e., zoogonidia, as in Protococcus, Ulothrix,
Oedogonium, Ectocarpus, etc., or with plant bodies similarly motile,
Bacteriaceae and Volvocaceae.
Page 280
265. Position of disseminule. The position on the plant of the organ to
be disseminated, i. e., its exposure to the distributing agent, plays a
considerable part in determining the degree of mobility. In the majority of
plants, the position of the inflorescence itself results in maximum exposure,
but in a large number of forms special modifications have been developed
for placing the spores or seeds in a more favorable position. In both cases,
there are often present also devices for bringing about the abscission of the
seed or fruit. It is, moreover, self-evident that the height of the inflorescence
above ground or above the surrounding vegetation is likewise of
considerable importance in increasing the trajectory. It is yet too early to
make a complete classification of contrivances for placing disseminules in
the most favorable exposure, but the following will serve as a basis for
future arrangements.
1. In all operculate Discomycetes, and especially in the Ascobolaceae,
where the asci project above the hymenium, the spores are raised above the
surface by tensions within the apothecium. This might be regarded as
dissemination by expulsion, if it were not for the fact that the spores fall
back into the cup, unless carried away by the wind.
2. In Gasteromycetes and in certain Hepaticae, the spores are not only
elevated slightly above the sporophore by the expanding capillitium or by
the mass of elaters, but they are also held apart in such a way that the wind
blows them out much more readily.
3. In Bryophyta, the sporophore regularly dehisces by a slit, or is
provided with a peristome. Both structures are for the purpose of sifting the
spores out into the wind; by reason of their hygroscopicity, they also insure
that the spores will not be shaken out in wet weather.
4. In a few grasses, such as Stipa and Aristida, the twisting and
intertwining of the awns lift the floret out of the glumes, and at the same
time constitute a contrivance readily blown away by the wind or carried by
attachment.
5. In certain Compositae, the involucral scales are reflexed at maturity,
and at the same time the disk becomes more or less convex, serving to
loosen the achenes. This result is also secured in certain species by the
drying and spreading of the pappus hairs.
6. The scapose Liguliflorae, Taraxacum, Agoseris, etc., are characterized
by the elongation of the scape after anthesis, with the result that the head is
be disseminated, i. e., its exposure to the distributing agent, plays a
considerable part in determining the degree of mobility. In the majority of
plants, the position of the inflorescence itself results in maximum exposure,
but in a large number of forms special modifications have been developed
for placing the spores or seeds in a more favorable position. In both cases,
there are often present also devices for bringing about the abscission of the
seed or fruit. It is, moreover, self-evident that the height of the inflorescence
above ground or above the surrounding vegetation is likewise of
considerable importance in increasing the trajectory. It is yet too early to
make a complete classification of contrivances for placing disseminules in
the most favorable exposure, but the following will serve as a basis for
future arrangements.
1. In all operculate Discomycetes, and especially in the Ascobolaceae,
where the asci project above the hymenium, the spores are raised above the
surface by tensions within the apothecium. This might be regarded as
dissemination by expulsion, if it were not for the fact that the spores fall
back into the cup, unless carried away by the wind.
2. In Gasteromycetes and in certain Hepaticae, the spores are not only
elevated slightly above the sporophore by the expanding capillitium or by
the mass of elaters, but they are also held apart in such a way that the wind
blows them out much more readily.
3. In Bryophyta, the sporophore regularly dehisces by a slit, or is
provided with a peristome. Both structures are for the purpose of sifting the
spores out into the wind; by reason of their hygroscopicity, they also insure
that the spores will not be shaken out in wet weather.
4. In a few grasses, such as Stipa and Aristida, the twisting and
intertwining of the awns lift the floret out of the glumes, and at the same
time constitute a contrivance readily blown away by the wind or carried by
attachment.
5. In certain Compositae, the involucral scales are reflexed at maturity,
and at the same time the disk becomes more or less convex, serving to
loosen the achenes. This result is also secured in certain species by the
drying and spreading of the pappus hairs.
6. The scapose Liguliflorae, Taraxacum, Agoseris, etc., are characterized
by the elongation of the scape after anthesis, with the result that the head is
Page 281
raised to a considerable height by the time the achenes are mature.
7. Carpotropic movements, though primarily for another purpose, often
serve to bring seeds and fruits into a better position for dissemination.
266. Seed production. The relation of spore or seed-production to
mobility is obvious in the case of mobile species; in the case of immobile
ones, it is just as evident that it has no effect, though it may still have
considerable influence in increasing migration. In the case of two species
with equally effective dissemination contrivances, the one with the largest
seed-production will be the more mobile. On the basis of the relation of
seeds to flower, two groups of plants may be distinguished, one,
Polyanthae, in which the flowers are many and the seeds few or single, as
in Compositae, and the other, Polyspermatae, Portulaca, Yucca, etc., in
which the number of seeds to each flower is large. So far as the actual
number of seeds produced is concerned, polyanthous plants may not differ
from polyspermatous ones, but, as a rule, they are much more highly
specialized for dissemination and are more mobile. The number of fertile
seeds is also much greater, a fact which is of great importance in ecesis, and
which, taken in connection with mobility, partially explains the supremacy
of the composites. Among the fungi and algae, the amount of spore-
production in a large degree determines the mobility, since these forms are
intrinsically permobile.
267. Agents of migration. In the last analysis, however, the possibility of
migration depends upon the action of distributive agents; in the absence of
these, even the most perfect contrivance is valueless, while their presence
brings about the distribution of the most immobile form. In short, migration
depends much more upon such agents than upon mobility, however perfect
the latter may be. It is, moreover, evident that the amount and extent of
migration will be determined primarily by the permanence and forcefulness
of the agent, as indicated by its ability to bring about transportation. Finally,
as will be shown later, the direction and rapidity of migration depend
directly upon the direction and intensity of the agent.
Migration results when spores, seeds, fruits, offshoots, or plants are
moved out of their home by water, wind, animals, man, gravity, glaciers,
growth, or mechanical propulsion. Corresponding to these agents, there
may be recognized the following groups:
7. Carpotropic movements, though primarily for another purpose, often
serve to bring seeds and fruits into a better position for dissemination.
266. Seed production. The relation of spore or seed-production to
mobility is obvious in the case of mobile species; in the case of immobile
ones, it is just as evident that it has no effect, though it may still have
considerable influence in increasing migration. In the case of two species
with equally effective dissemination contrivances, the one with the largest
seed-production will be the more mobile. On the basis of the relation of
seeds to flower, two groups of plants may be distinguished, one,
Polyanthae, in which the flowers are many and the seeds few or single, as
in Compositae, and the other, Polyspermatae, Portulaca, Yucca, etc., in
which the number of seeds to each flower is large. So far as the actual
number of seeds produced is concerned, polyanthous plants may not differ
from polyspermatous ones, but, as a rule, they are much more highly
specialized for dissemination and are more mobile. The number of fertile
seeds is also much greater, a fact which is of great importance in ecesis, and
which, taken in connection with mobility, partially explains the supremacy
of the composites. Among the fungi and algae, the amount of spore-
production in a large degree determines the mobility, since these forms are
intrinsically permobile.
267. Agents of migration. In the last analysis, however, the possibility of
migration depends upon the action of distributive agents; in the absence of
these, even the most perfect contrivance is valueless, while their presence
brings about the distribution of the most immobile form. In short, migration
depends much more upon such agents than upon mobility, however perfect
the latter may be. It is, moreover, evident that the amount and extent of
migration will be determined primarily by the permanence and forcefulness
of the agent, as indicated by its ability to bring about transportation. Finally,
as will be shown later, the direction and rapidity of migration depend
directly upon the direction and intensity of the agent.
Migration results when spores, seeds, fruits, offshoots, or plants are
moved out of their home by water, wind, animals, man, gravity, glaciers,
growth, or mechanical propulsion. Corresponding to these agents, there
may be recognized the following groups:
Page 282
1. Water, hydrochores. These comprise all plants distributed exclusively
by water, whether the latter acts as ocean currents, tides, streams, or surface
run-off. In the case of streams and run-off, especially, mobility plays little
part, provided the disseminules are impervious or little subject to injury by
water. Motile plants, or those with motile cells, which belong entirely to this
group, may be distinguished as autochores, which correspond closely to
mastigospores.
2. Wind, anemochores. This group includes the majority of all permobile
terrestrial plants, i. e., those in which modifications for increasing surface
have been carried to the extreme, or those which are already permobile by
reason of the minuteness of the spore or seed. Saccate, winged, comate,
parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits
represent the various types of modifications for wind-distribution.
3. Animals, zoochores. Among terrestrial plants, dissemination by
attachment represents essentially the same degree of specialization as is
found in wind-distributed plants. The three types of contrivances for this
purpose are found in spinose, hooked, and glandular fruits. Dissemination
by deglutition and by carriage, either intentional or unintentional, though of
less value, play a striking part on account of the great distance to which the
seeds may be carried. Dissemination by deglutition is characteristic of
sarcospores, and distribution by carriage of creatospores.
4. Man, brotochores. Dissemination by man has practically no connection
with mobility. It operates through great distances and over immense areas
as well as near at hand. It may be intentional, as in the case of cultivated
species, or unintentional, as in thousands of native or exotic species. No
other disseminating agent is comparable with man in respect to universal
and obvious migration.
5. Gravity, clitochores. The members of this group are exclusively
colline, montane, and alpine plants, growing on rocks, cliffs, and gravel
slides (talus), etc., in which the seeds reach lower positions merely by
falling, or more frequently by the breaking away and rolling down of rock
or soil masses and particles. Dissemination by this method is relatively
insignificant, though it plays an important part in the rock fields and gravel
slides of mountain regions, particularly in the case of immobile species.
6. Glaciers, crystallochores. At the present time, transport by glaciers is
of slight importance, because of the restriction of the latter to alpine and
by water, whether the latter acts as ocean currents, tides, streams, or surface
run-off. In the case of streams and run-off, especially, mobility plays little
part, provided the disseminules are impervious or little subject to injury by
water. Motile plants, or those with motile cells, which belong entirely to this
group, may be distinguished as autochores, which correspond closely to
mastigospores.
2. Wind, anemochores. This group includes the majority of all permobile
terrestrial plants, i. e., those in which modifications for increasing surface
have been carried to the extreme, or those which are already permobile by
reason of the minuteness of the spore or seed. Saccate, winged, comate,
parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits
represent the various types of modifications for wind-distribution.
3. Animals, zoochores. Among terrestrial plants, dissemination by
attachment represents essentially the same degree of specialization as is
found in wind-distributed plants. The three types of contrivances for this
purpose are found in spinose, hooked, and glandular fruits. Dissemination
by deglutition and by carriage, either intentional or unintentional, though of
less value, play a striking part on account of the great distance to which the
seeds may be carried. Dissemination by deglutition is characteristic of
sarcospores, and distribution by carriage of creatospores.
4. Man, brotochores. Dissemination by man has practically no connection
with mobility. It operates through great distances and over immense areas
as well as near at hand. It may be intentional, as in the case of cultivated
species, or unintentional, as in thousands of native or exotic species. No
other disseminating agent is comparable with man in respect to universal
and obvious migration.
5. Gravity, clitochores. The members of this group are exclusively
colline, montane, and alpine plants, growing on rocks, cliffs, and gravel
slides (talus), etc., in which the seeds reach lower positions merely by
falling, or more frequently by the breaking away and rolling down of rock
or soil masses and particles. Dissemination by this method is relatively
insignificant, though it plays an important part in the rock fields and gravel
slides of mountain regions, particularly in the case of immobile species.
6. Glaciers, crystallochores. At the present time, transport by glaciers is
of slight importance, because of the restriction of the latter to alpine and
Page 283
polar regions, where the flora is poorly developed. In the consideration of
migrations during the glacial epoch, however, it plays an important point.
7. Growth, blastochores. The mobility of species disseminated by
offshoots is extremely slight, and the annual movement relatively
insignificant. The certainty of migration and of ecesis, is, however, so great,
and the presence of offshoots so generally the rule in terrestrial plants that
growth plays an important part in migration, especially within formations.
8. Propulsion, bolochores. Like growth, dissemination by mechanical
propulsion, though operating through insignificant distances, exerts an
important effect in consequence of its cumulative action. The number of
plants, however, with contrivances for propulsion is very much smaller than
the number of blastochores. All bolochorous species agree in having
modifications by means of which a tension is established. At maturity, this
tension suddenly overcomes the resistance of sporangium or fruit, and
throws the enclosed spores or seeds to some distance from the parent plant.
In accordance with the manner in which the tension is produced, sling-fruits
may be classified as follows:
(a) Hygroscopicity, pladoboles. These include the ferns with annulate
sporangia, in which the expansion of the annulus by the absorption of
moisture bursts the sporangium more or less suddenly, though the actual
propulsion of the spores seems to come later as a result of dessication.
(b) Turgescence, edoboles. Dissemination by turgescence is highly
developed in Pilobolus and in Discomycetes, though in the latter
turgescence results rather in placing the spores in a position to be readily
carried by the wind. Impatiens and Oxalis furnish familiar examples of
fruits which dehisce in consequence of increased turgidity.
(c) Dessication, xerioboles. The number of fruits which dehisce upon
drying is very large, but only a small portion of these expel their seeds
forcibly. Geranium, Viola, Erysimum, and Lotus illustrate the different ways
in which dessication effects the sudden splitting of fruits.
(d) Resilience, tonoboles. In some plants, especially composites, labiates,
and borages, the achenes or nutlets are so placed in the persistent calyx or
involucre that the latter serves as a sort of mortar for projection, when the
stem of the plant is bent to one side by any force, such as the wind or an
animal. It will be noticed that two separate agents are actually concerned in
dissemination of this sort.
migrations during the glacial epoch, however, it plays an important point.
7. Growth, blastochores. The mobility of species disseminated by
offshoots is extremely slight, and the annual movement relatively
insignificant. The certainty of migration and of ecesis, is, however, so great,
and the presence of offshoots so generally the rule in terrestrial plants that
growth plays an important part in migration, especially within formations.
8. Propulsion, bolochores. Like growth, dissemination by mechanical
propulsion, though operating through insignificant distances, exerts an
important effect in consequence of its cumulative action. The number of
plants, however, with contrivances for propulsion is very much smaller than
the number of blastochores. All bolochorous species agree in having
modifications by means of which a tension is established. At maturity, this
tension suddenly overcomes the resistance of sporangium or fruit, and
throws the enclosed spores or seeds to some distance from the parent plant.
In accordance with the manner in which the tension is produced, sling-fruits
may be classified as follows:
(a) Hygroscopicity, pladoboles. These include the ferns with annulate
sporangia, in which the expansion of the annulus by the absorption of
moisture bursts the sporangium more or less suddenly, though the actual
propulsion of the spores seems to come later as a result of dessication.
(b) Turgescence, edoboles. Dissemination by turgescence is highly
developed in Pilobolus and in Discomycetes, though in the latter
turgescence results rather in placing the spores in a position to be readily
carried by the wind. Impatiens and Oxalis furnish familiar examples of
fruits which dehisce in consequence of increased turgidity.
(c) Dessication, xerioboles. The number of fruits which dehisce upon
drying is very large, but only a small portion of these expel their seeds
forcibly. Geranium, Viola, Erysimum, and Lotus illustrate the different ways
in which dessication effects the sudden splitting of fruits.
(d) Resilience, tonoboles. In some plants, especially composites, labiates,
and borages, the achenes or nutlets are so placed in the persistent calyx or
involucre that the latter serves as a sort of mortar for projection, when the
stem of the plant is bent to one side by any force, such as the wind or an
animal. It will be noticed that two separate agents are actually concerned in
dissemination of this sort.
Page 284
Frequently, two or more agents will act upon the same disseminule,
usually in succession. The possibility of such combinations in nature is
large, but actual cases seem to be infrequent, except where the activities of
man enter into the question. Some parts, moreover, such as awned
inflorescences, are carried almost equally well by wind or animals, and may
often be disseminated by the cooperation of these two agents. The wind also
often blows seeds and fruits into streams by which they are carried away,
but here again, parts adapted to wind-dissemination are injured as a rule by
immersion in water, and the number of plants capable of being scattered by
the successive action of wind and water is small.
In the present state of our knowledge of migration, it is impossible to
establish any definite correspondence between dissemination-contrivance,
agent, and habitat. As a general rule, plants growing in or near the water, in
so far as they are modified for this purpose at all, are adapted to water-
carriage. Species which grow in exposed grassy or barren habitats are for
the most part anemochores, while those that are found in the shelter of
forests and thickets are usually zoochorous, though the taller trees and
shrubs, being exposed to the upper air currents, are generally wind-
distributed. There is then a fair degree of correspondence, inasmuch as most
hydrophytes are hydrochorous, most hylophytes, zoochorous, and the
majority of poophytes and xerophytes, anemochorous. Definite conclusions
can be reached, however, only by the statistical study of representative
formations.
With respect to their activity, agents may be distinguished as constant, as
in the case of currents, streams, winds, slope, growth, and propulsion, or
intermittent, animals and man. In the former, the direction is more or less
determinate, and migration takes place year by year, i. e., it is continuous,
while in the latter dissemination is largely an accidental affair,
indeterminate in direction, and recurring only at indefinite intervals. The
effective conversion of migration into invasion is greatest when the
movement is continuous, and least when it is discontinuous, since, in the
latter, species are usually carried not only out of their particular habitat but
even far beyond their geographical area, and the migration, instead of being
an annual one with the possibility of gradual adjustment, may not recur for
several years, or may, indeed, never take place again. The rapidity of
migration is greatest in the case of intermittent agents, while the distance of
migration is variable, being great chiefly in the case of man, ocean currents,
usually in succession. The possibility of such combinations in nature is
large, but actual cases seem to be infrequent, except where the activities of
man enter into the question. Some parts, moreover, such as awned
inflorescences, are carried almost equally well by wind or animals, and may
often be disseminated by the cooperation of these two agents. The wind also
often blows seeds and fruits into streams by which they are carried away,
but here again, parts adapted to wind-dissemination are injured as a rule by
immersion in water, and the number of plants capable of being scattered by
the successive action of wind and water is small.
In the present state of our knowledge of migration, it is impossible to
establish any definite correspondence between dissemination-contrivance,
agent, and habitat. As a general rule, plants growing in or near the water, in
so far as they are modified for this purpose at all, are adapted to water-
carriage. Species which grow in exposed grassy or barren habitats are for
the most part anemochores, while those that are found in the shelter of
forests and thickets are usually zoochorous, though the taller trees and
shrubs, being exposed to the upper air currents, are generally wind-
distributed. There is then a fair degree of correspondence, inasmuch as most
hydrophytes are hydrochorous, most hylophytes, zoochorous, and the
majority of poophytes and xerophytes, anemochorous. Definite conclusions
can be reached, however, only by the statistical study of representative
formations.
With respect to their activity, agents may be distinguished as constant, as
in the case of currents, streams, winds, slope, growth, and propulsion, or
intermittent, animals and man. In the former, the direction is more or less
determinate, and migration takes place year by year, i. e., it is continuous,
while in the latter dissemination is largely an accidental affair,
indeterminate in direction, and recurring only at indefinite intervals. The
effective conversion of migration into invasion is greatest when the
movement is continuous, and least when it is discontinuous, since, in the
latter, species are usually carried not only out of their particular habitat but
even far beyond their geographical area, and the migration, instead of being
an annual one with the possibility of gradual adjustment, may not recur for
several years, or may, indeed, never take place again. The rapidity of
migration is greatest in the case of intermittent agents, while the distance of
migration is variable, being great chiefly in the case of man, ocean currents,
Page 285
and wind, and slight when the movement is due to slope, growth, or
propulsion. Disregarding the great distances over which artificial transport
may operate, seeds may be carried half way across the continent in a week
by strong-flying birds, while the possibilities of migration by growth or
expulsion are limited to a few inches, or at most to a few feet per year. This
slowness, however, is more than counterbalanced by the enormously greater
number of disseminules, and their much greater chance of becoming
established.
268. The direction of migration is determinate, except in the case of
those distributive agents which act constantly in the same direction. The
general tendency is, of course, forward, the lines of movement radiating in
all directions from the parent area. This is well illustrated by the operation
of winds which blow from any quarter. In the case of the constant winds,
migration takes a more or less definite direction, the latter being determined
to a large degree by the fruiting period of any particular species. In this
connection, it must be kept clearly in mind that the position of new areas
with reference to the original home of a species does not necessarily
indicate the direction of migration, as the disseminules may have been
carried to numerous other places in which ecesis was impossible. The local
distribution of zoochorous species is of necessity indeterminate, though
distant migration follows the pathways of migratory birds and animals. In
so far as dissemination by man takes place along great commercial routes,
or along highways, it is determinate. In ponds, lakes, and other bodies of
standing water, migration may occur in all directions, but in ocean currents,
streams, etc., the movement is determinate, except in the case of motile
species. The dissemination of plants by slopes, glaciers, etc., is local and
definite, while propulsion is in the highest degree indeterminate. Migration
by growth is equally indefinite, with the exception that hydrotropism and
chemotropism result in a radiate movement away from the mass, while
propulsion throws seeds indifferently into or away from the species-mass.
From the above it will be seen that distant migration may take place by
means of water, wind, animals or man, and, since all these agents act in a
more or less definite direction over great distances, that it will be in some
degree determinate. On the other hand, local migration will as regularly be
indeterminate, except in the case of streams and slopes. The direction of
migration, then, is controlled by these distributive agents, and the limit of
propulsion. Disregarding the great distances over which artificial transport
may operate, seeds may be carried half way across the continent in a week
by strong-flying birds, while the possibilities of migration by growth or
expulsion are limited to a few inches, or at most to a few feet per year. This
slowness, however, is more than counterbalanced by the enormously greater
number of disseminules, and their much greater chance of becoming
established.
268. The direction of migration is determinate, except in the case of
those distributive agents which act constantly in the same direction. The
general tendency is, of course, forward, the lines of movement radiating in
all directions from the parent area. This is well illustrated by the operation
of winds which blow from any quarter. In the case of the constant winds,
migration takes a more or less definite direction, the latter being determined
to a large degree by the fruiting period of any particular species. In this
connection, it must be kept clearly in mind that the position of new areas
with reference to the original home of a species does not necessarily
indicate the direction of migration, as the disseminules may have been
carried to numerous other places in which ecesis was impossible. The local
distribution of zoochorous species is of necessity indeterminate, though
distant migration follows the pathways of migratory birds and animals. In
so far as dissemination by man takes place along great commercial routes,
or along highways, it is determinate. In ponds, lakes, and other bodies of
standing water, migration may occur in all directions, but in ocean currents,
streams, etc., the movement is determinate, except in the case of motile
species. The dissemination of plants by slopes, glaciers, etc., is local and
definite, while propulsion is in the highest degree indeterminate. Migration
by growth is equally indefinite, with the exception that hydrotropism and
chemotropism result in a radiate movement away from the mass, while
propulsion throws seeds indifferently into or away from the species-mass.
From the above it will be seen that distant migration may take place by
means of water, wind, animals or man, and, since all these agents act in a
more or less definite direction over great distances, that it will be in some
degree determinate. On the other hand, local migration will as regularly be
indeterminate, except in the case of streams and slopes. The direction of
migration, then, is controlled by these distributive agents, and the limit of
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migration is determined by the intensity and duration of the agent, as well
as by the character of the space through which the latter operates.
as by the character of the space through which the latter operates.
Page 287
ECESIS
269. Concept. By the term ecesis is designated the series of phenomena
exhibited by an invading disseminule from the time it enters a new
formation until it becomes thoroughly established there. In a word, ecesis is
the adjustment of a plant to a new habitat. It comprises the whole process
covered more or less incompletely by acclimatization, naturalization,
accommodation, etc. It is the decisive factor in invasion, inasmuch as
migration is entirely ineffective without it, and is of great value in
indicating the presence and direction of migration in a great number of
species where the disseminule is too minute to be detected or too little
specialized to be recognizable.
The relation of migration to ecesis is a most intimate one: the latter
depends in a large measure upon the time, direction, rapidity, distance, and
amount of migration. In addition, there is an essential alternation between
the two, inasmuch as migration is followed by ecesis, and the latter then
establishes a new center from which further migration is possible, and so
on. The time of year in which fruits mature and distributive agents act has a
marked influence upon the establishment of a species. Disseminules
designed to pass through a resting period are often brought into conditions
where they germinate at once, and in which they perish because of
unfavorable physical factors, or because competing species are too far
advanced. On the other hand, spores and propagules designed for
immediate germination may be scattered abroad at a time when conditions
make growth impossible. The direction of movement is decisive in that the
seed or spore is carried into a habitat sufficiently like that of the parent to
secure establishment, or into one so dissimilar that germination is
impossible, or at least is not followed by growth and reproduction. The
rapidity and distance of migration have little influence, except upon the less
resistant disseminules, conidia, gemmae, etc. Finally, the amount of
migration, i. e., the number of migrants, is of the very greatest importance,
affecting directly the chances that vigorous disseminules will be carried into
places where ecesis is possible.
Normally, ecesis consists of three essential processes, germination,
growth, and reproduction. This is the rule among terrestrial plants, in which
269. Concept. By the term ecesis is designated the series of phenomena
exhibited by an invading disseminule from the time it enters a new
formation until it becomes thoroughly established there. In a word, ecesis is
the adjustment of a plant to a new habitat. It comprises the whole process
covered more or less incompletely by acclimatization, naturalization,
accommodation, etc. It is the decisive factor in invasion, inasmuch as
migration is entirely ineffective without it, and is of great value in
indicating the presence and direction of migration in a great number of
species where the disseminule is too minute to be detected or too little
specialized to be recognizable.
The relation of migration to ecesis is a most intimate one: the latter
depends in a large measure upon the time, direction, rapidity, distance, and
amount of migration. In addition, there is an essential alternation between
the two, inasmuch as migration is followed by ecesis, and the latter then
establishes a new center from which further migration is possible, and so
on. The time of year in which fruits mature and distributive agents act has a
marked influence upon the establishment of a species. Disseminules
designed to pass through a resting period are often brought into conditions
where they germinate at once, and in which they perish because of
unfavorable physical factors, or because competing species are too far
advanced. On the other hand, spores and propagules designed for
immediate germination may be scattered abroad at a time when conditions
make growth impossible. The direction of movement is decisive in that the
seed or spore is carried into a habitat sufficiently like that of the parent to
secure establishment, or into one so dissimilar that germination is
impossible, or at least is not followed by growth and reproduction. The
rapidity and distance of migration have little influence, except upon the less
resistant disseminules, conidia, gemmae, etc. Finally, the amount of
migration, i. e., the number of migrants, is of the very greatest importance,
affecting directly the chances that vigorous disseminules will be carried into
places where ecesis is possible.
Normally, ecesis consists of three essential processes, germination,
growth, and reproduction. This is the rule among terrestrial plants, in which
Page 288
migration regularly takes place by means of a resting part. In free aquatic
forms, however, the growing plant or part is usually disseminated, and
ecesis consists merely in being able to continue growth and to insure
reproduction. Here establishment is practically certain, on account of the
slight differences in aquatic habitats, excepting of course the extremes,
fresh water and salt water. The ease indeed with which migration and ecesis
are effected in the water often makes it impossible to speak properly of
invasion in this connection, since aquatics are to such a large extent
cosmopolitan. In dissemination by offshoots, the conditions are somewhat
similar. Here, also, ecesis comprises the sequence of growth and
reproduction, and invasion, in the sense of passing from one habitat to
another, is of rare occurrence, as the offshoot grows regularly under the
same conditions as the parent plant. The adjustment of growing plants and
parts is so slight, and their establishment so certain on account of their
inability to migrate into very remote or different habitats, that they may be
ignored in the following discussion.
In accordance with the above, it would be possible to distinguish three
groups of terrestrial plants: (1) those migrants which germinate and
disappear, (2) those which germinate and grow but never reproduce, (3)
those which reproduce, either by propagation or generation, or both. Such a
classification has little value, however, since the same species may behave
in all three fashions, depending upon the habitat to which it has migrated,
and since invasion does not occur unless the plant actually takes possession,
i. e., reproduces. From the latter statement, it follows that invasion occurs
only when a species migrates to a new place, in which it germinates,
matures, and reproduces. Maintenance by annual invasion simply, in which
the plants of each year disappear completely, can not then be regarded as
invasion proper. On the other hand, though such instances are rare, it is not
necessary that the invaders produce fruit, provided they are able to maintain
themselves, or to increase by propagation. Furthermore, if a plant
germinate, grow, and reproduce, it is relatively immaterial whether it persist
for a few years or for many, since, as we shall see under Succession, the
plants of one invasion are displaced by those of the next, the interval
between invasions increasing with the stabilization.
270. Germination of the seed. The germination of seed or spore is
determined by its viability and by the nature of the habitat. Viability
depends upon the structural characters of fruit, seed-coat, and endosperm,
forms, however, the growing plant or part is usually disseminated, and
ecesis consists merely in being able to continue growth and to insure
reproduction. Here establishment is practically certain, on account of the
slight differences in aquatic habitats, excepting of course the extremes,
fresh water and salt water. The ease indeed with which migration and ecesis
are effected in the water often makes it impossible to speak properly of
invasion in this connection, since aquatics are to such a large extent
cosmopolitan. In dissemination by offshoots, the conditions are somewhat
similar. Here, also, ecesis comprises the sequence of growth and
reproduction, and invasion, in the sense of passing from one habitat to
another, is of rare occurrence, as the offshoot grows regularly under the
same conditions as the parent plant. The adjustment of growing plants and
parts is so slight, and their establishment so certain on account of their
inability to migrate into very remote or different habitats, that they may be
ignored in the following discussion.
In accordance with the above, it would be possible to distinguish three
groups of terrestrial plants: (1) those migrants which germinate and
disappear, (2) those which germinate and grow but never reproduce, (3)
those which reproduce, either by propagation or generation, or both. Such a
classification has little value, however, since the same species may behave
in all three fashions, depending upon the habitat to which it has migrated,
and since invasion does not occur unless the plant actually takes possession,
i. e., reproduces. From the latter statement, it follows that invasion occurs
only when a species migrates to a new place, in which it germinates,
matures, and reproduces. Maintenance by annual invasion simply, in which
the plants of each year disappear completely, can not then be regarded as
invasion proper. On the other hand, though such instances are rare, it is not
necessary that the invaders produce fruit, provided they are able to maintain
themselves, or to increase by propagation. Furthermore, if a plant
germinate, grow, and reproduce, it is relatively immaterial whether it persist
for a few years or for many, since, as we shall see under Succession, the
plants of one invasion are displaced by those of the next, the interval
between invasions increasing with the stabilization.
270. Germination of the seed. The germination of seed or spore is
determined by its viability and by the nature of the habitat. Viability
depends upon the structural characters of fruit, seed-coat, and endosperm,
Page 289
and to a degree upon the nature of the protoplasm or embryo. The first three
affect the last directly, by protecting the embryo against dryness, against
injury due to carriage by water, or by deglutition, and probably in some
cases against excessive heat or cold. Marloth[33] has investigated the
structure of seed coats, establishing the following groups, which are
summarized here somewhat fully because of their bearing upon ecesis: (1)
seed coats without protective elements, endosperm absent or rudimentary,
Epilobium, Impatiens, Parnassia, Sagittaria, etc.; (2) protective elements
lacking or few, endosperm highly developed with thick-walled cells,
Liliaceae, Primulaceae, Rubiaceae, etc.; (3) protective cells present in the
seed coats, endosperm little or none, Boraginaceae, Crassulaceae,
Cruciferae, Labiatae, Papilionaceae, etc.; (4) protective elements present,
Asclepias, Campanula, Gentiana, Silene, Saxifraga, etc.; (5) protective cells
present, endosperm thick-walled, Euonymus, Helianthemum, Ribes. The
protective cells are of various kinds: (1) epidermal cells strongly
cuticularized, Caryophyllaceae, Crassulaceae, Fumariaceae,
Saxifragaceae; (2) parenchyma thick-walled, several-layered, Aesculus,
Castanea, Fagus; (3) parenchyma cells with the inner or radial walls
thickened, Campanula, Erythraea, Gentiana; (4) epidermal cells cup-
shaped, thick-walled, Cruciferae, Ribes, Vaccinium; (5) parenchyma with
thickened, cellulose walls, Geranium, Viburnum; (6) a single row of stone-
cells, Labiatae; (7) tissue of stone-cells, Hippuris, Naias, Potamogeton; (8)
elongate stone-cells, Coniferae, Cupuliferae, Euphorbia, Linum, Malva,
Viola; (9) short, columnar, thick-walled branched cells, Cucurbitaceae,
Datura, Hypericum; (10) prosenchyma with cellulose walls, Clematis; (11)
prosenchyma with lignified walls, Fraxinus, Rhamnus, Ranunculus. The
seed coats have a certain influence in determining germination at the proper
time, inasmuch as they make it difficult for the seed to germinate under the
stimulus of a quantity of warmth and moisture insufficient to support the
seedling. The effect of the endosperm, as well as that of other food supply
in the seed, upon germination and the establishment of the seedling is
obvious.
The behavior of seed or spore with respect to germination depends in a
large degree upon the character of the protoplasm or embryo, though in just
what way is at present a matter of conjecture. It is evident that many seeds
are not viable because fertilization has not been effected, and in
consequence no embryo has developed. This is the usual explanation of the
affect the last directly, by protecting the embryo against dryness, against
injury due to carriage by water, or by deglutition, and probably in some
cases against excessive heat or cold. Marloth[33] has investigated the
structure of seed coats, establishing the following groups, which are
summarized here somewhat fully because of their bearing upon ecesis: (1)
seed coats without protective elements, endosperm absent or rudimentary,
Epilobium, Impatiens, Parnassia, Sagittaria, etc.; (2) protective elements
lacking or few, endosperm highly developed with thick-walled cells,
Liliaceae, Primulaceae, Rubiaceae, etc.; (3) protective cells present in the
seed coats, endosperm little or none, Boraginaceae, Crassulaceae,
Cruciferae, Labiatae, Papilionaceae, etc.; (4) protective elements present,
Asclepias, Campanula, Gentiana, Silene, Saxifraga, etc.; (5) protective cells
present, endosperm thick-walled, Euonymus, Helianthemum, Ribes. The
protective cells are of various kinds: (1) epidermal cells strongly
cuticularized, Caryophyllaceae, Crassulaceae, Fumariaceae,
Saxifragaceae; (2) parenchyma thick-walled, several-layered, Aesculus,
Castanea, Fagus; (3) parenchyma cells with the inner or radial walls
thickened, Campanula, Erythraea, Gentiana; (4) epidermal cells cup-
shaped, thick-walled, Cruciferae, Ribes, Vaccinium; (5) parenchyma with
thickened, cellulose walls, Geranium, Viburnum; (6) a single row of stone-
cells, Labiatae; (7) tissue of stone-cells, Hippuris, Naias, Potamogeton; (8)
elongate stone-cells, Coniferae, Cupuliferae, Euphorbia, Linum, Malva,
Viola; (9) short, columnar, thick-walled branched cells, Cucurbitaceae,
Datura, Hypericum; (10) prosenchyma with cellulose walls, Clematis; (11)
prosenchyma with lignified walls, Fraxinus, Rhamnus, Ranunculus. The
seed coats have a certain influence in determining germination at the proper
time, inasmuch as they make it difficult for the seed to germinate under the
stimulus of a quantity of warmth and moisture insufficient to support the
seedling. The effect of the endosperm, as well as that of other food supply
in the seed, upon germination and the establishment of the seedling is
obvious.
The behavior of seed or spore with respect to germination depends in a
large degree upon the character of the protoplasm or embryo, though in just
what way is at present a matter of conjecture. It is evident that many seeds
are not viable because fertilization has not been effected, and in
consequence no embryo has developed. This is the usual explanation of the
Page 290
low germinating power of the seeds of some species, especially
polyspermatous ones. But even in viable seeds the behavior is always more
or less irregular. The seeds of some species will grow immediately after
ripening, while others germinate only after a resting period of uncertain
duration. The same is true of spores. Even in the case of seeds from the
same parent, under apparently similar conditions, while the majority will
germinate the first year, some will lie dormant for one or more years. The
precise reason why many seeds and spores germinate more readily after
being frozen is equally obscure. The period of time for which disseminules
may remain viable is extremely diverse, though, as would be expected, it is
much longer as a rule for seeds than for spores. The greater vitality of seeds
in the case of ruderal plants suggests that this diversity may be due simply
to variation in the vigor of the embryos. It would seem that under proper
conditions seeds may retain their viability for an indefinite period.
The influence of habitat upon germination is of primary importance,
though the manner in which its influence is exerted is by no means as
evident as might be supposed. In the case of seeds sown in the planthouse,
it is almost universally the case that germination is less than in nature,
notwithstanding the fact that temperature and moisture appear to be
optimum. In nature, the seeds of the species may be carried into a number
of different formations, any one or all of which may present conditions
unfavorable to germination. With respect to probability of germination,
habitats are of two sorts: those which are denuded and those which bear
vegetation. It is impossible to lay down general propositions with respect to
either group, since germination will vary with the character of the invading
species, the annual distribution of heat and moisture in the habitat, etc. In a
general way, however, it may be stated that the chances for germination are
greater in vegetation than in denuded areas, chiefly because the latter are
usually xerophytic. On the other hand, the lack of competition in the
denuded area tends to make ultimate establishment much more certain.
Here, as elsewhere when exact statistical results are desired, the use of the
quadrat, and especially of the permanent quadrat, is necessary to determine
the comparative germination of the invading species in relation to
denudation and vegetation.
271. Adjustment to the habitat. The seedling once established by
germination, the probability of its growing and maturing will depend upon
its habitat form, plasticity, and vegetation form. Even though it may
polyspermatous ones. But even in viable seeds the behavior is always more
or less irregular. The seeds of some species will grow immediately after
ripening, while others germinate only after a resting period of uncertain
duration. The same is true of spores. Even in the case of seeds from the
same parent, under apparently similar conditions, while the majority will
germinate the first year, some will lie dormant for one or more years. The
precise reason why many seeds and spores germinate more readily after
being frozen is equally obscure. The period of time for which disseminules
may remain viable is extremely diverse, though, as would be expected, it is
much longer as a rule for seeds than for spores. The greater vitality of seeds
in the case of ruderal plants suggests that this diversity may be due simply
to variation in the vigor of the embryos. It would seem that under proper
conditions seeds may retain their viability for an indefinite period.
The influence of habitat upon germination is of primary importance,
though the manner in which its influence is exerted is by no means as
evident as might be supposed. In the case of seeds sown in the planthouse,
it is almost universally the case that germination is less than in nature,
notwithstanding the fact that temperature and moisture appear to be
optimum. In nature, the seeds of the species may be carried into a number
of different formations, any one or all of which may present conditions
unfavorable to germination. With respect to probability of germination,
habitats are of two sorts: those which are denuded and those which bear
vegetation. It is impossible to lay down general propositions with respect to
either group, since germination will vary with the character of the invading
species, the annual distribution of heat and moisture in the habitat, etc. In a
general way, however, it may be stated that the chances for germination are
greater in vegetation than in denuded areas, chiefly because the latter are
usually xerophytic. On the other hand, the lack of competition in the
denuded area tends to make ultimate establishment much more certain.
Here, as elsewhere when exact statistical results are desired, the use of the
quadrat, and especially of the permanent quadrat, is necessary to determine
the comparative germination of the invading species in relation to
denudation and vegetation.
271. Adjustment to the habitat. The seedling once established by
germination, the probability of its growing and maturing will depend upon
its habitat form, plasticity, and vegetation form. Even though it may
Page 291
germinate under opposite conditions, a typical hylophyte, such as Impatiens
for example, will not thrive in an open meadow, nor will characteristic
poophytes, such as most grasses, grow in deep shade. In the same way,
xerophytes do not adapt themselves to hydrophytic habitats, nor
hydrophytes to xerophytic conditions. Many mesophytes, however, possess
to a certain degree the ability to adjust themselves to somewhat xerophytic
or hydrophytic situations, while woodland plants often invade either forest
or meadow. This capability for adjustment, i. e., plasticity, is greatest in
intermediate species, those that grow in habitats not characterized by great
excess or deficiency of some factor, and it is least in forms highly
specialized in respect to water-content, shade, etc. It may then be
established as a fundamental rule that ecesis is determined very largely by
the essential physical similarity of the old and the new habitat, except in the
case of plastic forms, which admit of a wider range of accommodation. The
plasticity of a plant is not necessarily indicated by structural modification,
though such adjustment is usually typical of plastic species, but it may
sometimes arise from a functional adaptation, which for some reason does
not produce concomitant structural changes. The former explains such
various habitat forms of the same species as are found in Galium boreale,
Gentiana acuta, etc., and the latter the morphological constancy of plants
like Chamaenerium, which grow in very diverse habitats.
The vegetation form of the invading species is often of the greatest
importance in determining whether it will become established. The
vegetation form represents those modifications which, produced in the
original home by competition, i. e., the struggle for existence, are primarily
of value in securing and maintaining a foothold. These comprise all
structures by means of which the plant occupies a definite space in the air,
through which the necessary light and heat reach it, and in the soil, from
which it draws its food supply. These structures are all organs of duration or
of perennation, such as root, rootstalk, bulb, tuber, woody stem, etc., which
find their greatest development among trees and shrubs, and their least
among annual herbs. But while the invaders are aided in securing
possession by the proper vegetation form, the occupation of the plant
already in possession is increased by the same means, and the outcome is
then largely determined by other factors. To avoid repetition, the bearing of
occupation upon invasion will be considered under succession.
for example, will not thrive in an open meadow, nor will characteristic
poophytes, such as most grasses, grow in deep shade. In the same way,
xerophytes do not adapt themselves to hydrophytic habitats, nor
hydrophytes to xerophytic conditions. Many mesophytes, however, possess
to a certain degree the ability to adjust themselves to somewhat xerophytic
or hydrophytic situations, while woodland plants often invade either forest
or meadow. This capability for adjustment, i. e., plasticity, is greatest in
intermediate species, those that grow in habitats not characterized by great
excess or deficiency of some factor, and it is least in forms highly
specialized in respect to water-content, shade, etc. It may then be
established as a fundamental rule that ecesis is determined very largely by
the essential physical similarity of the old and the new habitat, except in the
case of plastic forms, which admit of a wider range of accommodation. The
plasticity of a plant is not necessarily indicated by structural modification,
though such adjustment is usually typical of plastic species, but it may
sometimes arise from a functional adaptation, which for some reason does
not produce concomitant structural changes. The former explains such
various habitat forms of the same species as are found in Galium boreale,
Gentiana acuta, etc., and the latter the morphological constancy of plants
like Chamaenerium, which grow in very diverse habitats.
The vegetation form of the invading species is often of the greatest
importance in determining whether it will become established. The
vegetation form represents those modifications which, produced in the
original home by competition, i. e., the struggle for existence, are primarily
of value in securing and maintaining a foothold. These comprise all
structures by means of which the plant occupies a definite space in the air,
through which the necessary light and heat reach it, and in the soil, from
which it draws its food supply. These structures are all organs of duration or
of perennation, such as root, rootstalk, bulb, tuber, woody stem, etc., which
find their greatest development among trees and shrubs, and their least
among annual herbs. But while the invaders are aided in securing
possession by the proper vegetation form, the occupation of the plant
already in possession is increased by the same means, and the outcome is
then largely determined by other factors. To avoid repetition, the bearing of
occupation upon invasion will be considered under succession.
Page 292
BARRIERS
272. Concept. DeCandolle[34] seems to have been the first to use the term
barrier and to distinguish the various kinds, though Hedenberg[35] clearly
saw that stations of one kind were insurmountable obstacles to plants
belonging to a very different type. De Candolle pointed out that the natural
barriers to continuous invasion (“transport de proche en proche”) are: (1)
seas, which decrease invasion almost in inverse proportion to their extent;
(2) deserts; (3) mountain ranges, which are less absolute on account of
passes, valleys, etc.; (4) vegetation, marshes being barriers to dry land
plants, forests to those that fear the shade, etc. Grisebach[36], in discussing
the effect of barriers upon the constitution of vegetation, laid down the
fundamental rule that: “The supreme law which serves as the basis of the
permanent establishment of natural floras is to be recognized in the barriers
which have hindered or completely prevented invasion.”
Any feature of the topography, whether physical or biological, that
restricts or prevents invasion, is a barrier. Such features are usually
permanent and produce permanent barriers, though the latter may often be
temporary, existing for a few years only, or even for a single season. In this
last case, however, they are as a rule recurrent. Barriers may furthermore be
distinguished as complete or incomplete with respect to the thoroughness
with which they limit invasion. Finally, the consideration of this subject
gains clearness if it be recognized that there are barriers to migration as well
as to ecesis, and if we distinguish barriers as physical or biological with
reference to the character of the feature concerned.
273. Physical barriers are those in which limitation is produced by some
marked physiographic feature, such as the ocean or some other large body
of water, large rivers, mountain ranges and deserts (including ice and snow
fields). All of these are effective by virtue of their dominant physical
factors; hence they are barriers to the ecesis of species coming from very
different habitats, but they act as conductors for species from similar
vegetation, especially in the case of water currents. A body of water,
representing maximum water-content, is a barrier to mesophytic and
xerophytic species, but a conductor for hydrophytic ones; deserts set a limit
to the spread of mesophytic and hydrophytic plants, while they offer
conditions favorable to the invasion of xerophytes; and a high mountain
272. Concept. DeCandolle[34] seems to have been the first to use the term
barrier and to distinguish the various kinds, though Hedenberg[35] clearly
saw that stations of one kind were insurmountable obstacles to plants
belonging to a very different type. De Candolle pointed out that the natural
barriers to continuous invasion (“transport de proche en proche”) are: (1)
seas, which decrease invasion almost in inverse proportion to their extent;
(2) deserts; (3) mountain ranges, which are less absolute on account of
passes, valleys, etc.; (4) vegetation, marshes being barriers to dry land
plants, forests to those that fear the shade, etc. Grisebach[36], in discussing
the effect of barriers upon the constitution of vegetation, laid down the
fundamental rule that: “The supreme law which serves as the basis of the
permanent establishment of natural floras is to be recognized in the barriers
which have hindered or completely prevented invasion.”
Any feature of the topography, whether physical or biological, that
restricts or prevents invasion, is a barrier. Such features are usually
permanent and produce permanent barriers, though the latter may often be
temporary, existing for a few years only, or even for a single season. In this
last case, however, they are as a rule recurrent. Barriers may furthermore be
distinguished as complete or incomplete with respect to the thoroughness
with which they limit invasion. Finally, the consideration of this subject
gains clearness if it be recognized that there are barriers to migration as well
as to ecesis, and if we distinguish barriers as physical or biological with
reference to the character of the feature concerned.
273. Physical barriers are those in which limitation is produced by some
marked physiographic feature, such as the ocean or some other large body
of water, large rivers, mountain ranges and deserts (including ice and snow
fields). All of these are effective by virtue of their dominant physical
factors; hence they are barriers to the ecesis of species coming from very
different habitats, but they act as conductors for species from similar
vegetation, especially in the case of water currents. A body of water,
representing maximum water-content, is a barrier to mesophytic and
xerophytic species, but a conductor for hydrophytic ones; deserts set a limit
to the spread of mesophytic and hydrophytic plants, while they offer
conditions favorable to the invasion of xerophytes; and a high mountain
Page 293
range, because of the reduction of temperature, restricts the extension of
macrothermal and mesothermal plants. A mountain range, unlike other
physical barriers, is also an obstacle to migration, inasmuch as natural
distributive agents rarely act through it or over it.
274. Biological barriers include vegetation, man and animals, and plant
parasites. The limiting effect of vegetation is exhibited in two ways. In the
first place, a formation acts as a barrier to the ecesis of species invading it
from the formations of another type, on account of the physical differences
of the habitats. Whether such a barrier be complete or partial will depend
upon the degree of dissimilarity existing between the formations.
Hylophytes are unable to invade a prairie, though open thicket plants may
do so to a certain degree. In the same way, a forest formation on account of
its diffuse light is a barrier to poophytes; and a swamp, because of the
amount and character of the water-content, sets a limit to both hylophytes
and poophytes. Formations, such as forests, thickets, etc., sometimes act
also as direct obstacles to migration, as in the case of tumbleweeds and
other anemochores, clitochores, etc. A marked effect of vegetation in
decreasing invasion arises from the closed association typical of stable
formations and of social exclusive species. In these, the occupation is so
thorough and the struggle for existence so intense that the invaders, though
fitted to grow under the physical factors present, are unable to compete with
the species in possession for the requisite amount of some necessary factor.
Closed associations usually act as complete barriers, while open ones
restrict invasion in direct proportion to the degree of occupation. To this
fact may be traced a fundamental law of succession, viz., the number of
stages in a succession is determined largely by the increasing difficulty of
invasion as the habitat becomes stabilized. Man and animals affect
migration directly, though not obviously, by the destruction of disseminules.
They operate as a pronounced barrier to ecesis wherever they alter
conditions in such a way as to make them unfavorable to invading species,
or when, by direct action upon the latter, such as grazing, tramping,
parasitism, etc., they turn the scale in the struggle for existence. The
absence of insects adapted to insure fertilization is sometimes a serious
barrier to the establishment of adventitious or introduced plants. The
presence of parasitic fungi, in so far as they destroy the seeds of plants, acts
as an obstacle to migration, and restricts or prevents ecesis in so far as the
macrothermal and mesothermal plants. A mountain range, unlike other
physical barriers, is also an obstacle to migration, inasmuch as natural
distributive agents rarely act through it or over it.
274. Biological barriers include vegetation, man and animals, and plant
parasites. The limiting effect of vegetation is exhibited in two ways. In the
first place, a formation acts as a barrier to the ecesis of species invading it
from the formations of another type, on account of the physical differences
of the habitats. Whether such a barrier be complete or partial will depend
upon the degree of dissimilarity existing between the formations.
Hylophytes are unable to invade a prairie, though open thicket plants may
do so to a certain degree. In the same way, a forest formation on account of
its diffuse light is a barrier to poophytes; and a swamp, because of the
amount and character of the water-content, sets a limit to both hylophytes
and poophytes. Formations, such as forests, thickets, etc., sometimes act
also as direct obstacles to migration, as in the case of tumbleweeds and
other anemochores, clitochores, etc. A marked effect of vegetation in
decreasing invasion arises from the closed association typical of stable
formations and of social exclusive species. In these, the occupation is so
thorough and the struggle for existence so intense that the invaders, though
fitted to grow under the physical factors present, are unable to compete with
the species in possession for the requisite amount of some necessary factor.
Closed associations usually act as complete barriers, while open ones
restrict invasion in direct proportion to the degree of occupation. To this
fact may be traced a fundamental law of succession, viz., the number of
stages in a succession is determined largely by the increasing difficulty of
invasion as the habitat becomes stabilized. Man and animals affect
migration directly, though not obviously, by the destruction of disseminules.
They operate as a pronounced barrier to ecesis wherever they alter
conditions in such a way as to make them unfavorable to invading species,
or when, by direct action upon the latter, such as grazing, tramping,
parasitism, etc., they turn the scale in the struggle for existence. The
absence of insects adapted to insure fertilization is sometimes a serious
barrier to the establishment of adventitious or introduced plants. The
presence of parasitic fungi, in so far as they destroy the seeds of plants, acts
as an obstacle to migration, and restricts or prevents ecesis in so far as the
Page 294
fungi destroy the invaders, or place them at a disadvantage in the struggle
for existence.
275. Influence of barriers. Physical barriers are typically permanent in
character, while biological ones are either permanent or temporary,
depending upon the permanence of the formation and the constancy of the
physical factors which determine it. A stable formation, such as a forest or
meadow, which acts as a decided barrier to invasion from adjacent
vegetation, may disappear completely, as a result of a landslide, flood, or
burn, or through the activity of man, and may leave an area into which
invaders crowd from every point. Often, without undergoing marked
change, a formation which has presented conditions unfavorable to the
ecesis of species of mesophytic character may, by reason of a temporary
change in climate, become sufficiently modified to permit the invasion of
mesophytes. On the other hand, a meadow ceases to be a barrier to prairie
xerophytes during a period of unusually dry years. A peculiar example of
the modification of a barrier is afforded by the defoliation of aspen forests
in the mountains as a result of which poophytes have been enabled to
invade them. Nearly all xerophytic stretches of sand and gravel, dunes,
blowouts, gravel slides, etc., and even prairies to a certain degree, exhibit a
recurrent seasonal change in spring, as a result of which the hot, dry surface
becomes sufficiently moist to permit the germination and growth of
invaders, which are entirely barred out during the remainder of the year. In
an absolute sense, no barrier is complete, since the coldest as well as the
driest portions of the earth’s surface are capable, at times at least, of
supporting the lowest types of vegetation. Relatively, however, in
connection with the natural spread of terrestrial plants, it is possible to
distinguish partial barriers from complete ones. Such a distinction is of
importance in the consideration of invasions from a definite region, as it is
only in this restricted sense that complete barriers have produced
endemism.
Distance, though hardly to be considered a barrier in the strict sense of
the word, unquestionably plays an important part in determining the amount
of invasion. The effect of distance is best seen in the case of migration, as it
influences ecesis only in those rare cases where viability is affected. The
importance of distance, or take the converse, of proximity, is readily
ascertained by the study of any succession from denudation. It has been
established that the contiguous vegetation furnishes 75–90 per cent of the
for existence.
275. Influence of barriers. Physical barriers are typically permanent in
character, while biological ones are either permanent or temporary,
depending upon the permanence of the formation and the constancy of the
physical factors which determine it. A stable formation, such as a forest or
meadow, which acts as a decided barrier to invasion from adjacent
vegetation, may disappear completely, as a result of a landslide, flood, or
burn, or through the activity of man, and may leave an area into which
invaders crowd from every point. Often, without undergoing marked
change, a formation which has presented conditions unfavorable to the
ecesis of species of mesophytic character may, by reason of a temporary
change in climate, become sufficiently modified to permit the invasion of
mesophytes. On the other hand, a meadow ceases to be a barrier to prairie
xerophytes during a period of unusually dry years. A peculiar example of
the modification of a barrier is afforded by the defoliation of aspen forests
in the mountains as a result of which poophytes have been enabled to
invade them. Nearly all xerophytic stretches of sand and gravel, dunes,
blowouts, gravel slides, etc., and even prairies to a certain degree, exhibit a
recurrent seasonal change in spring, as a result of which the hot, dry surface
becomes sufficiently moist to permit the germination and growth of
invaders, which are entirely barred out during the remainder of the year. In
an absolute sense, no barrier is complete, since the coldest as well as the
driest portions of the earth’s surface are capable, at times at least, of
supporting the lowest types of vegetation. Relatively, however, in
connection with the natural spread of terrestrial plants, it is possible to
distinguish partial barriers from complete ones. Such a distinction is of
importance in the consideration of invasions from a definite region, as it is
only in this restricted sense that complete barriers have produced
endemism.
Distance, though hardly to be considered a barrier in the strict sense of
the word, unquestionably plays an important part in determining the amount
of invasion. The effect of distance is best seen in the case of migration, as it
influences ecesis only in those rare cases where viability is affected. The
importance of distance, or take the converse, of proximity, is readily
ascertained by the study of any succession from denudation. It has been
established that the contiguous vegetation furnishes 75–90 per cent of the
Page 295
constituent species of the initial formation, and in mountainous regions,
where ruderal plants are extremely rare, the percentage is even higher. The
reason for this is to be found not only in the fact that the adjacent species
have a much shorter distance to go, and hence will be carried in much
greater quantity, but also in that the species of the formations beyond must
pass through or over the adjacent ones. In the latter case, the number of
disseminules is relatively small on account of the distance, while invasion
through the intermediate vegetation, if not entirely impossible, is extremely
slow, so that plants coming in by this route reach the denuded area only to
find it already occupied. It is as yet impossible to give a definite numerical
value to proximity in the various invasions that mark any particular
succession. This will not be feasible until a satisfactory method has been
found for determining a coefficient of mobility, but, this once done, it will
be a relatively simple matter, not merely to trace the exact evolution of any
succession of formations, but actually to ascertain from the adjacent
vegetation the probable constitution of a particular future stage.
From what has been said, it follows that the primary effect of barriers
upon vegetation is obstruction. Where the barrier is in the pathway of
migration, however, it causes deflection of the migrant as a rule, and sets up
migration in a new direction. This is often the case when the strong winds
of the plains carry disseminules towards the mountains and, being unable to
cross the range, drop them at the base, or, being deflected, carry them away
at right angles to the original direction. The same thing happens when
resistant fruits and seeds borne by the wind fall into streams of water or into
ocean currents. The direction of migration is changed, and what is normally
a barrier serves as an agent of dissemination.
ENDEMISM
276. Concept. Since its first use by DeCandolle, the term endemic has
been employed quite consistently by phytogeographers with the meaning of
“peculiar to a certain region.” Some confusion, however, has arisen from
the fact that a few authors have made it more or less synonymous with
indigenous and autochthonous, while others have regarded it as an antonym
of exotic. In its proper sense, endemic refers to distribution, and not to
origin. Its exact opposite will be found then in Fenzl’s term polydemic,
where ruderal plants are extremely rare, the percentage is even higher. The
reason for this is to be found not only in the fact that the adjacent species
have a much shorter distance to go, and hence will be carried in much
greater quantity, but also in that the species of the formations beyond must
pass through or over the adjacent ones. In the latter case, the number of
disseminules is relatively small on account of the distance, while invasion
through the intermediate vegetation, if not entirely impossible, is extremely
slow, so that plants coming in by this route reach the denuded area only to
find it already occupied. It is as yet impossible to give a definite numerical
value to proximity in the various invasions that mark any particular
succession. This will not be feasible until a satisfactory method has been
found for determining a coefficient of mobility, but, this once done, it will
be a relatively simple matter, not merely to trace the exact evolution of any
succession of formations, but actually to ascertain from the adjacent
vegetation the probable constitution of a particular future stage.
From what has been said, it follows that the primary effect of barriers
upon vegetation is obstruction. Where the barrier is in the pathway of
migration, however, it causes deflection of the migrant as a rule, and sets up
migration in a new direction. This is often the case when the strong winds
of the plains carry disseminules towards the mountains and, being unable to
cross the range, drop them at the base, or, being deflected, carry them away
at right angles to the original direction. The same thing happens when
resistant fruits and seeds borne by the wind fall into streams of water or into
ocean currents. The direction of migration is changed, and what is normally
a barrier serves as an agent of dissemination.
ENDEMISM
276. Concept. Since its first use by DeCandolle, the term endemic has
been employed quite consistently by phytogeographers with the meaning of
“peculiar to a certain region.” Some confusion, however, has arisen from
the fact that a few authors have made it more or less synonymous with
indigenous and autochthonous, while others have regarded it as an antonym
of exotic. In its proper sense, endemic refers to distribution, and not to
origin. Its exact opposite will be found then in Fenzl’s term polydemic,
Page 296
dwelling in several regions. Indigenous (autochthonous) and exotic, on the
contrary, denote origin, and are antonyms, indigenous signifying native, and
exotic foreign. As Drude has shown, endemic plants may be either
indigenous, as in the case of those species that have never moved out of the
original habitat, or exotic, as in the much rarer instances where a polydemic
species has disappeared from its original home and from all regions into
which it has migrated except one. It is understood that not all indigenous or
exotic species are endemic. The proportion of endemic to polydemic
species is a variable and somewhat artificial one, depending upon the size
of the divisions employed.
277. Causes. The primary causes of endemism are two, lack of migration
and presence of barriers. Since distributive agents are practically universal,
lack of migration corresponds essentially to immobility, a fact which
decreases the difficulty of ascertaining the immediate causes of endemism
in any particular species. Either immobility or a barrier may produce
endemism; extremely immobile plants, for example, liliaceous species
propagating almost wholly by underground parts, are as a rule endemic,
while alpine plants and those of oceanic islands are endemic in the highest
degree, regardless of their mobility. When the two conditions act
concomitantly upon a species, endemism is almost inevitable. It can not be
supposed, however, that immobility or natural barriers alone, or the
concomitance of the two, must invariably give rise to endemic species; the
most immobile plant may be carried into another region by unusual or
accidental agencies, or the most formidable barrier to migration may be
overcome by the intensity of an agent or through the action of man.
Endemism is also brought about by the modification of species; new or
nascent species are as a rule endemic. Whether they will remain endemic or
not will depend upon the perfection of their contrivances for dissemination
and upon the presence of barriers to migration or ecesis. Finally, as Drude
was the first to point out, the disappearance of a polydemic species in all
regions but one, owing to the struggle for existence or to changed physical
conditions, will result in endemism.
278. Significance. Endemism is readily recognized by methods of
distributional statistics, applied to areas limited by natural barriers to
migration or ecesis. For political areas, it has no significance whatever,
unless the boundaries of these coincide with barriers. It determines in the
first degree the validity of regions, though the latter are often recognized
contrary, denote origin, and are antonyms, indigenous signifying native, and
exotic foreign. As Drude has shown, endemic plants may be either
indigenous, as in the case of those species that have never moved out of the
original habitat, or exotic, as in the much rarer instances where a polydemic
species has disappeared from its original home and from all regions into
which it has migrated except one. It is understood that not all indigenous or
exotic species are endemic. The proportion of endemic to polydemic
species is a variable and somewhat artificial one, depending upon the size
of the divisions employed.
277. Causes. The primary causes of endemism are two, lack of migration
and presence of barriers. Since distributive agents are practically universal,
lack of migration corresponds essentially to immobility, a fact which
decreases the difficulty of ascertaining the immediate causes of endemism
in any particular species. Either immobility or a barrier may produce
endemism; extremely immobile plants, for example, liliaceous species
propagating almost wholly by underground parts, are as a rule endemic,
while alpine plants and those of oceanic islands are endemic in the highest
degree, regardless of their mobility. When the two conditions act
concomitantly upon a species, endemism is almost inevitable. It can not be
supposed, however, that immobility or natural barriers alone, or the
concomitance of the two, must invariably give rise to endemic species; the
most immobile plant may be carried into another region by unusual or
accidental agencies, or the most formidable barrier to migration may be
overcome by the intensity of an agent or through the action of man.
Endemism is also brought about by the modification of species; new or
nascent species are as a rule endemic. Whether they will remain endemic or
not will depend upon the perfection of their contrivances for dissemination
and upon the presence of barriers to migration or ecesis. Finally, as Drude
was the first to point out, the disappearance of a polydemic species in all
regions but one, owing to the struggle for existence or to changed physical
conditions, will result in endemism.
278. Significance. Endemism is readily recognized by methods of
distributional statistics, applied to areas limited by natural barriers to
migration or ecesis. For political areas, it has no significance whatever,
unless the boundaries of these coincide with barriers. It determines in the
first degree the validity of regions, though the latter are often recognized
Page 297
also by the presence of barriers and by the character of the vegetation.
Endemism may occur in areas of vegetation of any rank from a formation to
a zone. When the term is not qualified, however, it should be used of
species with reference to formations alone. Comparisons to be of value,
however, can be instituted only between areas of the same order, i. e.,
between two or more formations, two or more regions, provinces, etc. In the
same way, taxonomic groups of the same rank should be used in such
comparisons, i. e., species should be contrasted with species, genera with
genera, and families with families, except when it is desired to obtain some
measure of the age of the vegetation by the differentiation of the endemic
phyla within it. There will be seen to exist a fundamental correspondence
between the rank of the floral division and the taxonomic group, though the
apparent exceptions to this are still too numerous to warrant its expression
in a general law. As a rule, however, formations most frequently show
endemic habitat forms and species, more rarely endemic genera; regions
and provinces commonly exhibit endemic species and genera, rarely
endemic families; while zones and hemispheres contain endemic orders as
well as families. This correspondence is readily seen to depend primarily
upon the fact that increased differentiation in the taxonomic sense is a
concomitant of the increased invasion of endemic species, measured in
terms of distance and difference in habitat.
It is too early to decide satisfactorily whether it is proper to speak of
formations as endemic. At first thought it would seem that all formations,
with the exception of ruderal ones, were endemic, but a study of almost any
transition area between regions would seem to point to the opposite
conclusion, viz., that no formations are properly endemic. It is equally
impossible at present to distinguish different types of endemics, such as
relictae, etc., as any such classification must await the elaboration of a
method for determining the phylogeny of a natural group of species by an
investigation of their comparative differentiation in connection with their
migration in all directions from the vegetation center into new habitats. In
short, it will not be possible to make a thorough study of endemism and to
postulate its laws until modern methods of research have been extended to a
much larger portion of the vegetation of the globe. The final task of
phytogeography is the division of the earth’s vegetation into natural areas. It
will be at once evident that most plants can not properly be called endemic
until the natural regions in which they are found have been accurately
Endemism may occur in areas of vegetation of any rank from a formation to
a zone. When the term is not qualified, however, it should be used of
species with reference to formations alone. Comparisons to be of value,
however, can be instituted only between areas of the same order, i. e.,
between two or more formations, two or more regions, provinces, etc. In the
same way, taxonomic groups of the same rank should be used in such
comparisons, i. e., species should be contrasted with species, genera with
genera, and families with families, except when it is desired to obtain some
measure of the age of the vegetation by the differentiation of the endemic
phyla within it. There will be seen to exist a fundamental correspondence
between the rank of the floral division and the taxonomic group, though the
apparent exceptions to this are still too numerous to warrant its expression
in a general law. As a rule, however, formations most frequently show
endemic habitat forms and species, more rarely endemic genera; regions
and provinces commonly exhibit endemic species and genera, rarely
endemic families; while zones and hemispheres contain endemic orders as
well as families. This correspondence is readily seen to depend primarily
upon the fact that increased differentiation in the taxonomic sense is a
concomitant of the increased invasion of endemic species, measured in
terms of distance and difference in habitat.
It is too early to decide satisfactorily whether it is proper to speak of
formations as endemic. At first thought it would seem that all formations,
with the exception of ruderal ones, were endemic, but a study of almost any
transition area between regions would seem to point to the opposite
conclusion, viz., that no formations are properly endemic. It is equally
impossible at present to distinguish different types of endemics, such as
relictae, etc., as any such classification must await the elaboration of a
method for determining the phylogeny of a natural group of species by an
investigation of their comparative differentiation in connection with their
migration in all directions from the vegetation center into new habitats. In
short, it will not be possible to make a thorough study of endemism and to
postulate its laws until modern methods of research have been extended to a
much larger portion of the vegetation of the globe. The final task of
phytogeography is the division of the earth’s vegetation into natural areas. It
will be at once evident that most plants can not properly be called endemic
until the natural regions in which they are found have been accurately
Page 298
defined, a work which has barely begun. In the much simpler matter of
distribution, upon which the accuracy of statistical methods depends
directly, there are few regions sufficiently well known at the present time to
yield anything like permanent results.
POLYPHYLESIS AND POLYGENESIS
279. Concept. The idea of polyphylesis, as advanced by Engler, contains
two distinct concepts: (1) that a species may arise in two different places or
at two different times from the same species, and (2) that a genus or higher
group may arise at different places or times by the convergence of two or
more lines of origin. It is here proposed to restrict polyphylesis, as its
meaning would indicate, to the second concept, and to employ for the first
the term polygenesis,[37] first suggested by Huxley in the sense of
polyphylesis. The term polyphylesis is extended, however, to cover the
origin of those species which arise at different places or times from the
convergence of two or more different species, a logical extension of the idea
underlying polyphyletic genera, though it may seem at first thought to be
absurd. Polygenesis may be formally defined as the origin of one species
from another species at two or more distinct places on the earth’s surface, at
the same time or at different times, or its origin in the same place at
different times. Polyphylesis, on the contrary, is the origin of one species
from two or more different species at different places, at the same time or at
different times. It is evident that what is true of species in this connection
will hold equally well of genera and higher groups. Opposed to polygenesis
is monogenesis, in which a species arises but once from another species;
with polyphylesis is to be contrasted monophylesis, in which the species
arises from a single other species. It will be noticed at once that these two
concepts are closely related. The following diagrams will serve to make the
above distinctions more evident:
distribution, upon which the accuracy of statistical methods depends
directly, there are few regions sufficiently well known at the present time to
yield anything like permanent results.
POLYPHYLESIS AND POLYGENESIS
279. Concept. The idea of polyphylesis, as advanced by Engler, contains
two distinct concepts: (1) that a species may arise in two different places or
at two different times from the same species, and (2) that a genus or higher
group may arise at different places or times by the convergence of two or
more lines of origin. It is here proposed to restrict polyphylesis, as its
meaning would indicate, to the second concept, and to employ for the first
the term polygenesis,[37] first suggested by Huxley in the sense of
polyphylesis. The term polyphylesis is extended, however, to cover the
origin of those species which arise at different places or times from the
convergence of two or more different species, a logical extension of the idea
underlying polyphyletic genera, though it may seem at first thought to be
absurd. Polygenesis may be formally defined as the origin of one species
from another species at two or more distinct places on the earth’s surface, at
the same time or at different times, or its origin in the same place at
different times. Polyphylesis, on the contrary, is the origin of one species
from two or more different species at different places, at the same time or at
different times. It is evident that what is true of species in this connection
will hold equally well of genera and higher groups. Opposed to polygenesis
is monogenesis, in which a species arises but once from another species;
with polyphylesis is to be contrasted monophylesis, in which the species
arises from a single other species. It will be noticed at once that these two
concepts are closely related. The following diagrams will serve to make the
above distinctions more evident:
Page 299
I. Polygenesis II. Polyphylesis III. Monogenesis (Monophylesis)
In I, a species A, becomes scattered over a large area in a series of places,
m ... mn, with the same physical factors, in any or all of which may arise the
new species a. In II, a species with xerophytic tendency, A, and one with
mesophytic tendency, B, in the course of migration find themselves
respectively in a more mesophytic habitat, m, and a more xerophytic one, x,
in which either may give rise to the new form, c, which is more or less
intermediate between A and B. In III, the method of origin is of the simplest
type, in which a species is modified directly into another one, or is split up
into several.
280. Proofs of polygenesis. In affirming the probability of a polygenetic
origin of species, there is no intention of asserting that all species originate
in this way. It seems evident that a very large number of species of
restricted range are certainly monogenetic, at least as far as origin in space
is concerned. It is possible that any species may arise at two or more
distinct times. Polygenesis can occur readily only in species of more or less
extensive area, in which recur instances of the same or similar habitat. The
relative frequence and importance of the two methods can hardly be
conjectured as yet, but origin by monogenesis would seem to be the rule.
The arguments adduced by Engler in support of polygenesis are in
themselves conclusive, but the investigations of the past decade have
brought to light additional proofs, especially from the experimental side. In
In I, a species A, becomes scattered over a large area in a series of places,
m ... mn, with the same physical factors, in any or all of which may arise the
new species a. In II, a species with xerophytic tendency, A, and one with
mesophytic tendency, B, in the course of migration find themselves
respectively in a more mesophytic habitat, m, and a more xerophytic one, x,
in which either may give rise to the new form, c, which is more or less
intermediate between A and B. In III, the method of origin is of the simplest
type, in which a species is modified directly into another one, or is split up
into several.
280. Proofs of polygenesis. In affirming the probability of a polygenetic
origin of species, there is no intention of asserting that all species originate
in this way. It seems evident that a very large number of species of
restricted range are certainly monogenetic, at least as far as origin in space
is concerned. It is possible that any species may arise at two or more
distinct times. Polygenesis can occur readily only in species of more or less
extensive area, in which recur instances of the same or similar habitat. The
relative frequence and importance of the two methods can hardly be
conjectured as yet, but origin by monogenesis would seem to be the rule.
The arguments adduced by Engler in support of polygenesis are in
themselves conclusive, but the investigations of the past decade have
brought to light additional proofs, especially from the experimental side. In
Page 300
determining the physical factors of prairie and mountain formations, and
especially by methods of experimental ecology, the author has found that
habitats are much less complex than they are ordinarily thought to be, since
water-content and humidity, and to a less degree light, constitute the only
factors which produce direct modification. In addition, it has been
ascertained that the minimum difference of water-content, humidity, or
light, necessary to produce a distinguishable morphological adjustment is
much greater than the unit differences recorded by the instruments. In short,
the differences of habitats, as ascertained by thermograph, psychrometer
and photometer, are much greater than their efficient differences, and, with
respect to their ability to produce modification, habitats fall into relatively
few categories. A striking illustration of this is seen in the superficially very
different habitats, desert, strand, alkali plain, alpine moor, and arctic tundra,
all of which are capable of producing the same type of xerophyte. It follows
from this that many more or less plastic species of extensive geographical
area will find themselves in similar or identical situations, measured in
terms of efficient differences, and will be modified in the same way in two
or more of these. In mountain regions, where interruption of the surface and
consequent alternation are great, the mutual invasion of contiguous
formations is of frequent occurrence, often resulting in habitat forms. The
spots in which these nascent species, such as Galium boreale hylocolum,
Aster levis lochmocolus, etc., are found, are often so related to the area of
the parent species as to demonstrate conclusively that these forms are the
result of polygenesis and not of migration. Naturally, what is true of a small
area will hold equally well of a large region, and the recurrence of the same
habitat form may be accepted as conclusive proof of polygenesis. The most
convincing evidences of multiple origin, however, are to be found in what
De Vries has called “mutations.” It makes little difference whether we
accept mutations in the exact sense of this author, or regard them as forms
characterized by latent variability. The evidence is conclusive that the same
form may arise in nature or in cultivation, in Holland or in America, not
merely once, but several or many times. In the presence of such
confirmation, it is unnecessary to accumulate proofs. Polygenesis throws a
new light upon many difficult problems of invasion and distribution, and, as
a working principle, admits of repeated tests in the field. It obviates,
moreover, the almost insuperable difficulties in the way of explaining the
distribution of many polygenetic species on the basis of migration alone.
especially by methods of experimental ecology, the author has found that
habitats are much less complex than they are ordinarily thought to be, since
water-content and humidity, and to a less degree light, constitute the only
factors which produce direct modification. In addition, it has been
ascertained that the minimum difference of water-content, humidity, or
light, necessary to produce a distinguishable morphological adjustment is
much greater than the unit differences recorded by the instruments. In short,
the differences of habitats, as ascertained by thermograph, psychrometer
and photometer, are much greater than their efficient differences, and, with
respect to their ability to produce modification, habitats fall into relatively
few categories. A striking illustration of this is seen in the superficially very
different habitats, desert, strand, alkali plain, alpine moor, and arctic tundra,
all of which are capable of producing the same type of xerophyte. It follows
from this that many more or less plastic species of extensive geographical
area will find themselves in similar or identical situations, measured in
terms of efficient differences, and will be modified in the same way in two
or more of these. In mountain regions, where interruption of the surface and
consequent alternation are great, the mutual invasion of contiguous
formations is of frequent occurrence, often resulting in habitat forms. The
spots in which these nascent species, such as Galium boreale hylocolum,
Aster levis lochmocolus, etc., are found, are often so related to the area of
the parent species as to demonstrate conclusively that these forms are the
result of polygenesis and not of migration. Naturally, what is true of a small
area will hold equally well of a large region, and the recurrence of the same
habitat form may be accepted as conclusive proof of polygenesis. The most
convincing evidences of multiple origin, however, are to be found in what
De Vries has called “mutations.” It makes little difference whether we
accept mutations in the exact sense of this author, or regard them as forms
characterized by latent variability. The evidence is conclusive that the same
form may arise in nature or in cultivation, in Holland or in America, not
merely once, but several or many times. In the presence of such
confirmation, it is unnecessary to accumulate proofs. Polygenesis throws a
new light upon many difficult problems of invasion and distribution, and, as
a working principle, admits of repeated tests in the field. It obviates,
moreover, the almost insuperable difficulties in the way of explaining the
distribution of many polygenetic species on the basis of migration alone.
Page 301
281. Origin by polyphylesis. In 1898, the author first advanced a
tentative hypothesis to the effect that a species homogeneous
morphologically may arise from two distinct though related species. During
subsequent years of formational study, the conviction has grown in regard
to the probability of such a method of origin. Since the appearance of
Engler’s work, a polyphyletic origin for certain genera has been very
generally accepted by botanists, but all have ignored the fact that the
polyphylesis of genera carries with it the admission of such origin for
species, since the former are merely groups of the latter. I can not, however,
agree with Engler, that polyphyletic genera, and hence species also, are
necessarily unnatural. If the convergence of the lines of polyphylesis has
been great, resulting in essential morphological harmony, the genus is a
natural one, even though the ancestral phyla may be recognizable. If, on the
other hand, the convergence is more or less imperfect, resulting in
subgroups of species more nearly related within the groups than between
them, the genus can hardly be termed natural. This condition may, however,
prevail in a monophyletic genus with manifest divergence and still not be an
indication that it is artificial.
Darwin[38], in speaking of convergence, has said: “If two species,
belonging to two distinct though allied genera, had both produced a large
number of new and divergent forms, it is conceivable that these might
approach each other so closely that they would have all to be classified
under the same genus; and thus the descendants of two distinct genera
would converge into one.” The application of this statement to species
would at once show the possibility of polyphylesis in the latter, and a
further examination of the matter will demonstrate its probability. It is
perfectly evident that a species may be split into two or more forms by
varying the conditions, let us say of water-content, and that the descendants
of these forms may again be changed into the parent type by reversing the
process. This has, in fact, been done experimentally. Since it is admittedly
impossible to draw any absolute line between forms, varieties, and species,
it is at once clear that two distinct though related species, especially if they
are plastic, may be caused to converge in such a way that the variants may
constitute a new and homogeneous species. This may be illustrated by a
concrete case at present under investigation. Kuhnistera purpurea differs
from K. candida in being smaller, in having fewer, smaller, and more
narrow leaflets, and a globoid spike of purple flowers in place of an
tentative hypothesis to the effect that a species homogeneous
morphologically may arise from two distinct though related species. During
subsequent years of formational study, the conviction has grown in regard
to the probability of such a method of origin. Since the appearance of
Engler’s work, a polyphyletic origin for certain genera has been very
generally accepted by botanists, but all have ignored the fact that the
polyphylesis of genera carries with it the admission of such origin for
species, since the former are merely groups of the latter. I can not, however,
agree with Engler, that polyphyletic genera, and hence species also, are
necessarily unnatural. If the convergence of the lines of polyphylesis has
been great, resulting in essential morphological harmony, the genus is a
natural one, even though the ancestral phyla may be recognizable. If, on the
other hand, the convergence is more or less imperfect, resulting in
subgroups of species more nearly related within the groups than between
them, the genus can hardly be termed natural. This condition may, however,
prevail in a monophyletic genus with manifest divergence and still not be an
indication that it is artificial.
Darwin[38], in speaking of convergence, has said: “If two species,
belonging to two distinct though allied genera, had both produced a large
number of new and divergent forms, it is conceivable that these might
approach each other so closely that they would have all to be classified
under the same genus; and thus the descendants of two distinct genera
would converge into one.” The application of this statement to species
would at once show the possibility of polyphylesis in the latter, and a
further examination of the matter will demonstrate its probability. It is
perfectly evident that a species may be split into two or more forms by
varying the conditions, let us say of water-content, and that the descendants
of these forms may again be changed into the parent type by reversing the
process. This has, in fact, been done experimentally. Since it is admittedly
impossible to draw any absolute line between forms, varieties, and species,
it is at once clear that two distinct though related species, especially if they
are plastic, may be caused to converge in such a way that the variants may
constitute a new and homogeneous species. This may be illustrated by a
concrete case at present under investigation. Kuhnistera purpurea differs
from K. candida in being smaller, in having fewer, smaller, and more
narrow leaflets, and a globoid spike of purple flowers in place of an
Page 302
elongated one of white flowers; in a word, it is more xerophytic. This
conclusion is completely corroborated by its occurrence. On dozens of
slopes examined, Kuhnistera purpurea has never been found mingling with
K. candida on lower slopes, except where an accident of the surface has
resulted in a local decrease of water-content. The experiment as conducted
is a simple one, consisting merely in sowing seed of each in the zone of the
other, and in growing K. purpurea under controlled mesophytic conditions,
and K. candida under similarly measured xerophytic conditions in the
planthouse.
While the polyphyletic origin of species is in a fair way to be decided by
experiment, it receives support from several well-known phenomena. The
striking similarity in the plant body of families taxonomically so distinct as
the Cactaceae, Stapeliaceae, and Euphorbiaceae, or Cyperaceae and
Juncaceae, indicates that a vegetation form may be polyphyletic. On the
other hand, the local appearance of zygomorphy, of symphysis, and of
aphanisis in the floral types of phylogeneticallv distinct families is a proof
of the operation of convergence in reproductive characters. To be sure, the
convergence is never so great as to produce more than superficial similarity,
but this is because the groups are markedly different in so many
fundamental characters. The same tendency in closely related species would
easily result in identity. As in the case of polygenesis, the relatively small
number of typically distinct habitats makes it clear that two different
species of wide distribution, bearing to each other the relations of xerophyte
to mesophyte, of hydrophyte to mesophyte, or of poophyte to hylophyte,
might often find themselves in reciprocal situations, with the result that they
would give rise to the same new form. The final proof of the polyphylesis
of species is afforded by the experiments of De Vries in mutation. De Vries
found that Oenothera nanella arose from O. Lamarckiana, O. laevifolia,
and O. scintillans; Oenothera scintillans arose from O. lata and O.
Lamarckiana; Oenothera rubrinervis from O. Lamarckiana, O. laevifolia,
O. lata, O. oblonga, O. nanella, and O. scintillans, etc. Whatever may be
the rank assigned to these mutations, whether form, variety, or species,
there can be no question of their polyphyletic origin, nor, in consequence of
the connection of mutations with variations through such inconstant forms
as O. scintillans, O. elliptica, and O. sublinearis, of the possibility of
polyphylesis in any two distinct though related species or genera.
conclusion is completely corroborated by its occurrence. On dozens of
slopes examined, Kuhnistera purpurea has never been found mingling with
K. candida on lower slopes, except where an accident of the surface has
resulted in a local decrease of water-content. The experiment as conducted
is a simple one, consisting merely in sowing seed of each in the zone of the
other, and in growing K. purpurea under controlled mesophytic conditions,
and K. candida under similarly measured xerophytic conditions in the
planthouse.
While the polyphyletic origin of species is in a fair way to be decided by
experiment, it receives support from several well-known phenomena. The
striking similarity in the plant body of families taxonomically so distinct as
the Cactaceae, Stapeliaceae, and Euphorbiaceae, or Cyperaceae and
Juncaceae, indicates that a vegetation form may be polyphyletic. On the
other hand, the local appearance of zygomorphy, of symphysis, and of
aphanisis in the floral types of phylogeneticallv distinct families is a proof
of the operation of convergence in reproductive characters. To be sure, the
convergence is never so great as to produce more than superficial similarity,
but this is because the groups are markedly different in so many
fundamental characters. The same tendency in closely related species would
easily result in identity. As in the case of polygenesis, the relatively small
number of typically distinct habitats makes it clear that two different
species of wide distribution, bearing to each other the relations of xerophyte
to mesophyte, of hydrophyte to mesophyte, or of poophyte to hylophyte,
might often find themselves in reciprocal situations, with the result that they
would give rise to the same new form. The final proof of the polyphylesis
of species is afforded by the experiments of De Vries in mutation. De Vries
found that Oenothera nanella arose from O. Lamarckiana, O. laevifolia,
and O. scintillans; Oenothera scintillans arose from O. lata and O.
Lamarckiana; Oenothera rubrinervis from O. Lamarckiana, O. laevifolia,
O. lata, O. oblonga, O. nanella, and O. scintillans, etc. Whatever may be
the rank assigned to these mutations, whether form, variety, or species,
there can be no question of their polyphyletic origin, nor, in consequence of
the connection of mutations with variations through such inconstant forms
as O. scintillans, O. elliptica, and O. sublinearis, of the possibility of
polyphylesis in any two distinct though related species or genera.
Page 303
KINDS OF INVASION
282. Continuous and intermittent invasion. With respect to the
frequency of migration, we may distinguish invasion as continuous, or
intermittent. Continuous invasion, which is indeed usually mutual, occurs
between contiguous formations of more or less similar character, in which
there is an annual movement from one into the other, and at the same time a
forward movement through each, resulting from the invaders established
the preceding year. By far the greater amount of invasion is of this sort, as
may readily be seen from the fact that migration varies inversely as the
distance, and ecesis may decrease even more rapidly than the distance
increases. The significant feature of continuous invasion is that an outpost
may be reinforced every year, thus making probable the establishment of
new outposts from this as a center, and the ultimate extension of the species
over a wide area. The comparatively short distance and the regular
alternation of migration and ecesis render invasion of this sort very
effective. An excellent illustration of this is seen in transition areas and
regions, which are due directly to continuous and usually to mutual
invasion. Intermittent invasion results commonly from distant carriage,
though it may occur very rarely between dissimilar adjacent formations,
when a temporary swing in the physical factors makes ecesis possible for a
time. It is characterized by the fact that the succession of factors which have
brought about the invasion is more or less accidental and may never recur.
Intermittent invasion is relatively rare, and from the small number of
disseminules affected, it is of little importance in modifying vegetation
quantitatively. On the other hand, since a species may often be carried far
from its geographical area, it is frequently of great significance in
distribution.
283. Complete and partial invasion. When the movement of invaders
into a formation is so great that the original occupants are finally driven out,
the invasion may be termed complete. Such invasion is found regularly in
the case of many ruderal formations, and is typical of the later stages of
many successions. It is ordinarily the result of continuous invasion. If the
number of invaders is sufficiently small that they may be adopted into the
formation without radically changing the latter, the invasion is partial. This
is doubtless true of the greater number of invasions, though these are
282. Continuous and intermittent invasion. With respect to the
frequency of migration, we may distinguish invasion as continuous, or
intermittent. Continuous invasion, which is indeed usually mutual, occurs
between contiguous formations of more or less similar character, in which
there is an annual movement from one into the other, and at the same time a
forward movement through each, resulting from the invaders established
the preceding year. By far the greater amount of invasion is of this sort, as
may readily be seen from the fact that migration varies inversely as the
distance, and ecesis may decrease even more rapidly than the distance
increases. The significant feature of continuous invasion is that an outpost
may be reinforced every year, thus making probable the establishment of
new outposts from this as a center, and the ultimate extension of the species
over a wide area. The comparatively short distance and the regular
alternation of migration and ecesis render invasion of this sort very
effective. An excellent illustration of this is seen in transition areas and
regions, which are due directly to continuous and usually to mutual
invasion. Intermittent invasion results commonly from distant carriage,
though it may occur very rarely between dissimilar adjacent formations,
when a temporary swing in the physical factors makes ecesis possible for a
time. It is characterized by the fact that the succession of factors which have
brought about the invasion is more or less accidental and may never recur.
Intermittent invasion is relatively rare, and from the small number of
disseminules affected, it is of little importance in modifying vegetation
quantitatively. On the other hand, since a species may often be carried far
from its geographical area, it is frequently of great significance in
distribution.
283. Complete and partial invasion. When the movement of invaders
into a formation is so great that the original occupants are finally driven out,
the invasion may be termed complete. Such invasion is found regularly in
the case of many ruderal formations, and is typical of the later stages of
many successions. It is ordinarily the result of continuous invasion. If the
number of invaders is sufficiently small that they may be adopted into the
formation without radically changing the latter, the invasion is partial. This
is doubtless true of the greater number of invasions, though these are
Page 304
regularly much less striking and important than instances of complete
invasion.
Fig. 59. Continuous invasion into a new area; mats of Arenaria sajanensis. Silene acaulis and
Sieversia turbinata invading an alpine gravel slide.
284. Permanent and temporary invasion. The permanence of invasion
depends upon the success attending ecesis, and upon the stability of the
formation. It has already been noticed that under certain conditions plants
may germinate and grow, and if they are perennials, even become
established, and still ecesis be so imperfect that reproduction is impossible.
Others may find the conditions sufficiently favorable for propagation, but
unfavorable for the formation of flowers and fruits. Finally there are plants
which seem to be perfectly established for a few years, only to disappear
completely. The latter are examples of temporary invasion. It is necessary
invasion.
Fig. 59. Continuous invasion into a new area; mats of Arenaria sajanensis. Silene acaulis and
Sieversia turbinata invading an alpine gravel slide.
284. Permanent and temporary invasion. The permanence of invasion
depends upon the success attending ecesis, and upon the stability of the
formation. It has already been noticed that under certain conditions plants
may germinate and grow, and if they are perennials, even become
established, and still ecesis be so imperfect that reproduction is impossible.
Others may find the conditions sufficiently favorable for propagation, but
unfavorable for the formation of flowers and fruits. Finally there are plants
which seem to be perfectly established for a few years, only to disappear
completely. The latter are examples of temporary invasion. It is necessary
Page 305
to draw clearly the line between complete and partial invasion in this
connection. The former is temporary in the initial or intermediate stages of
nearly all successions, as compared with the ultimate stages, though it is in
a large degree permanent in comparison with the partial invasion of species
which are able to maintain themselves for a few years. In a sense, there is a
real distinction between the two, inasmuch as a particular stage of
succession is permanent as long as the habitat remains essentially the same.
A critical study of the species of such stages shows, however, that they
manifest very different degrees of permanence. Species which invade stable
vegetation temporarily have been termed adventive by A. DeCandolle.
Permanent invasion occurs when a species becomes permanently
established in a more or less stable formation. It is characteristic of the great
majority of invaders found in the grassland and forest stages of successions.
Plants which have arisen within a formation or have been a constituent
part of it since its origin are indigenous. Contrasted with these are the
species which have invaded the formation since it received its distinctive
impress: these are derived. The determination of the indigenous and derived
species of a formation or larger division is of the utmost importance, as it
enables us to retrace the steps by which the formation has reached its
present structure, and to reconstruct formations long since disappeared. To
render it less difficult, it is necessary to scrutinize the derived elements
closely, first, because it is easiest to recognize the indigenous species by
eliminating the derived, and second, because this analysis will show that not
all derived species have entered the formation at the same time and from the
same sources. Derived species may be termed vicine, when they are fully
established invaders from adjacent formations or regions, and adventitious,
when they have come from distant formations and have succeeded in
establishing themselves. Finally, those derived species which are unable to
establish themselves permanently are adventive.
MANNER OF INVASION
285. Entrance into the habitat. Since the ecesis of invaders depends in
large measure upon the occupation of the plants in possession, the method
and degree of invasion will be determined by the presence or absence of
vegetation. Areas without vegetation are either originally naked or denuded,
connection. The former is temporary in the initial or intermediate stages of
nearly all successions, as compared with the ultimate stages, though it is in
a large degree permanent in comparison with the partial invasion of species
which are able to maintain themselves for a few years. In a sense, there is a
real distinction between the two, inasmuch as a particular stage of
succession is permanent as long as the habitat remains essentially the same.
A critical study of the species of such stages shows, however, that they
manifest very different degrees of permanence. Species which invade stable
vegetation temporarily have been termed adventive by A. DeCandolle.
Permanent invasion occurs when a species becomes permanently
established in a more or less stable formation. It is characteristic of the great
majority of invaders found in the grassland and forest stages of successions.
Plants which have arisen within a formation or have been a constituent
part of it since its origin are indigenous. Contrasted with these are the
species which have invaded the formation since it received its distinctive
impress: these are derived. The determination of the indigenous and derived
species of a formation or larger division is of the utmost importance, as it
enables us to retrace the steps by which the formation has reached its
present structure, and to reconstruct formations long since disappeared. To
render it less difficult, it is necessary to scrutinize the derived elements
closely, first, because it is easiest to recognize the indigenous species by
eliminating the derived, and second, because this analysis will show that not
all derived species have entered the formation at the same time and from the
same sources. Derived species may be termed vicine, when they are fully
established invaders from adjacent formations or regions, and adventitious,
when they have come from distant formations and have succeeded in
establishing themselves. Finally, those derived species which are unable to
establish themselves permanently are adventive.
MANNER OF INVASION
285. Entrance into the habitat. Since the ecesis of invaders depends in
large measure upon the occupation of the plants in possession, the method
and degree of invasion will be determined by the presence or absence of
vegetation. Areas without vegetation are either originally naked or denuded,
Page 306
while vegetation with respect to the degree of occupation is open
(sporadophytia), or closed (pycnophytia). Each type of area presents
different conditions to invaders, largely with respect to the factors
determining ecesis. Naked habitats, rocks, talus, gravel slides, and dunes,
while they offer ample opportunity for invasion on account of the lack of
occupation, are really invaded with the greatest difficulty, not only because
they contain originally few or no disseminules, but also because of their
xerophytic character and the difficulty of obtaining a foothold, on account
of the extreme density or instability of the soil. Denuded habitats, blowouts,
sand draws, ponds, flood plains, wastes, fields, and burns, usually afford
maximum opportunity for invasion. They invariably contain a large number
of disseminules ready to spring up as soon as the original vegetation is
destroyed. The surface, moreover, is usually such as to catch disseminules
and to offer them optimum conditions of moisture and nutrition. Open
formations are readily invaded, though the increased occupation renders
entrance more difficult than it is in denuded areas. Closed formations, on
the other hand, are characterized by a minimum of invasion, partly because
invaders from different formations find unfavorable conditions in them, but
chiefly because the occupation of the inhabitants is so complete that
invaders are unable to establish themselves.
Invasion takes place by the penetration of single individuals or groups of
individuals. This will depend in the first place upon the character of the
disseminule. It is evident that, no matter how numerous the achenes may be,
the invasion of those anemochorous species with comate or winged seeds or
one-seeded fruits will be of the first type, while all species in which the
disseminule is a several or many-seeded fruit or plant, as in hooked fruits,
tumbleweeds, etc., will tend to produce a group of invaders. Occasionally of
course, the accidents of migration will bring together a few one-seeded
disseminules into a group, or will scatter the seeds of a many-seeded fruit,
but these constitute relatively rare exceptions. This distinction in the matter
of invasion is of value in studying the relative rapidity of the latter, and the
establishment of new centers, but it is of greatest importance in explaining
the historical arrangement of species in a formation, and hence has a direct
bearing upon alternation. It is entirely independent of the number of
invaders, which, as we have seen, depends upon seed-production, mobility,
distance, occupation, etc., but is based solely upon mode of arrangement,
and will be found to underlie the primary types of abundance, copious, and
(sporadophytia), or closed (pycnophytia). Each type of area presents
different conditions to invaders, largely with respect to the factors
determining ecesis. Naked habitats, rocks, talus, gravel slides, and dunes,
while they offer ample opportunity for invasion on account of the lack of
occupation, are really invaded with the greatest difficulty, not only because
they contain originally few or no disseminules, but also because of their
xerophytic character and the difficulty of obtaining a foothold, on account
of the extreme density or instability of the soil. Denuded habitats, blowouts,
sand draws, ponds, flood plains, wastes, fields, and burns, usually afford
maximum opportunity for invasion. They invariably contain a large number
of disseminules ready to spring up as soon as the original vegetation is
destroyed. The surface, moreover, is usually such as to catch disseminules
and to offer them optimum conditions of moisture and nutrition. Open
formations are readily invaded, though the increased occupation renders
entrance more difficult than it is in denuded areas. Closed formations, on
the other hand, are characterized by a minimum of invasion, partly because
invaders from different formations find unfavorable conditions in them, but
chiefly because the occupation of the inhabitants is so complete that
invaders are unable to establish themselves.
Invasion takes place by the penetration of single individuals or groups of
individuals. This will depend in the first place upon the character of the
disseminule. It is evident that, no matter how numerous the achenes may be,
the invasion of those anemochorous species with comate or winged seeds or
one-seeded fruits will be of the first type, while all species in which the
disseminule is a several or many-seeded fruit or plant, as in hooked fruits,
tumbleweeds, etc., will tend to produce a group of invaders. Occasionally of
course, the accidents of migration will bring together a few one-seeded
disseminules into a group, or will scatter the seeds of a many-seeded fruit,
but these constitute relatively rare exceptions. This distinction in the matter
of invasion is of value in studying the relative rapidity of the latter, and the
establishment of new centers, but it is of greatest importance in explaining
the historical arrangement of species in a formation, and hence has a direct
bearing upon alternation. It is entirely independent of the number of
invaders, which, as we have seen, depends upon seed-production, mobility,
distance, occupation, etc., but is based solely upon mode of arrangement,
and will be found to underlie the primary types of abundance, copious, and
Page 307
gregarious. In this connection, it should also be noted that the contingencies
of migration, especially the concomitant action in the same direction of two
or more distributive agencies, often results in the penetration of a group of
individuals belonging to two or more species. This may well be termed
mass invasion; it is characteristic of transition areas or regions, and along
valleys or other natural routes for migration it gives rise to species guilds.
The movement of species guilds constitutes one of the most complex and
interesting problems in the whole field of invasion, the solution of which
can be attempted only after the thorough analysis of the simpler invasions
between formations. A better understanding of the meaning of invasion by
species guilds is imperative for the natural limitation of regions, as at
present such groups constitute alien associations in many regions otherwise
homogeneous.
286. Influence of levels. The invasion of a formation may occur at three
different levels: (1) at the level of the facies, (2) below the facies, (3) above
the facies, depending directly upon the relative height of invaders and
occupants. The invasion level is an extremely simple matter to determine,
except in the case of woody plants, such as shrubs and trees, which attain
their average height only after many years. Its importance is fundamental.
The level at which invasion occurs not only determines the immediate
constitution of the formation, whether its impress shall still be given by the
occupants or by the invaders or by both together, but it also decides the
whole future of the formation, i. e., whether the invaders or occupants shall
persist unmodified or modified. The problem is an extremely complex one,
but the careful analysis of invasion at each level throws a flood of light
upon it. The entrance of invaders of the same general height as the facies of
a formation results regularly in mixed formations. This is well illustrated by
the structure of the transition areas between two formations of the same
category, i. e., forests, meadows, etc. It is seldom, however, that the facies
and invaders are so equally matched in height and other qualities that they
remain in equilibrium for a long period. One or the other has a slight
advantage in height, or the one suffers shading or crowding better than the
other, is longer-lived or faster-growing, with the result that invader yields to
occupant, or occupant to invader. It is a well-known fact that many mixed
formations represent intermediate stages of development.
Invasion at a level different from that of the facies is inevitably followed
by modification. If the invasion takes place below the facies, the invaders
of migration, especially the concomitant action in the same direction of two
or more distributive agencies, often results in the penetration of a group of
individuals belonging to two or more species. This may well be termed
mass invasion; it is characteristic of transition areas or regions, and along
valleys or other natural routes for migration it gives rise to species guilds.
The movement of species guilds constitutes one of the most complex and
interesting problems in the whole field of invasion, the solution of which
can be attempted only after the thorough analysis of the simpler invasions
between formations. A better understanding of the meaning of invasion by
species guilds is imperative for the natural limitation of regions, as at
present such groups constitute alien associations in many regions otherwise
homogeneous.
286. Influence of levels. The invasion of a formation may occur at three
different levels: (1) at the level of the facies, (2) below the facies, (3) above
the facies, depending directly upon the relative height of invaders and
occupants. The invasion level is an extremely simple matter to determine,
except in the case of woody plants, such as shrubs and trees, which attain
their average height only after many years. Its importance is fundamental.
The level at which invasion occurs not only determines the immediate
constitution of the formation, whether its impress shall still be given by the
occupants or by the invaders or by both together, but it also decides the
whole future of the formation, i. e., whether the invaders or occupants shall
persist unmodified or modified. The problem is an extremely complex one,
but the careful analysis of invasion at each level throws a flood of light
upon it. The entrance of invaders of the same general height as the facies of
a formation results regularly in mixed formations. This is well illustrated by
the structure of the transition areas between two formations of the same
category, i. e., forests, meadows, etc. It is seldom, however, that the facies
and invaders are so equally matched in height and other qualities that they
remain in equilibrium for a long period. One or the other has a slight
advantage in height, or the one suffers shading or crowding better than the
other, is longer-lived or faster-growing, with the result that invader yields to
occupant, or occupant to invader. It is a well-known fact that many mixed
formations represent intermediate stages of development.
Invasion at a level different from that of the facies is inevitably followed
by modification. If the invasion takes place below the facies, the invaders
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will be exterminated gradually, or slowly assimilated. In either case, there is
little structural change in the formation, and its stability is affected slightly
or not at all. If the invaders overtop the facies in any considerable number,
the entire formation undergoes partial or complete modification, or in
extreme cases it disappears, as is typically the case in succession. A peculiar
variation of invasion at a level above the facies is seen where woody plants
invade grassland, when the trees or shrubs become more or less uniformly
scattered in an open woodland or open thicket. Here the grassland takes on
an altogether different appearance superficially, though it is usually
unchanged, except beneath and about the invaders, where either adaptation
or extermination results. Finally, it should be borne in mind that the
invasion of a particular formation, especially in the case of layered thickets
and forests, often takes place at two levels, at the height of the facies and
below the facies.
INVESTIGATION OF INVASION
287. The methods to be used in the study of invasion are those already
described elsewhere. The migration circle is of the first importance because
it makes it possible to secure an accurate record of actual movement.
Quadrat and transect are valuable, but from their nature they are more
serviceable for ecesis than for migration. All of these should be of the
permanent type, in order that the fate of invaders may be followed for
several years at least. Permanent areas furnish evidence of the changes
wrought in the actual vegetation, while denuded ones can serve only to
show the potential migration and ecesis of the constituent species.
Transition zones and areas are special seats of invasion; they are best
studied by means of the belt transect and the ecotone chart. The movement
of a line of invaders or of scattered outposts is traced by the use of labeled
stakes at the points concerned. It is clear that this method will yield
conclusive data in regard to the great invasions between regions, such as the
movement of species guilds, the advance of the forest frontier, etc. When
invasion is scattered, factor instruments can not be used to advantage, but
where the invading line is well marked, or where extra-formational areas
occur, a knowledge of the physical factors is a great aid.
little structural change in the formation, and its stability is affected slightly
or not at all. If the invaders overtop the facies in any considerable number,
the entire formation undergoes partial or complete modification, or in
extreme cases it disappears, as is typically the case in succession. A peculiar
variation of invasion at a level above the facies is seen where woody plants
invade grassland, when the trees or shrubs become more or less uniformly
scattered in an open woodland or open thicket. Here the grassland takes on
an altogether different appearance superficially, though it is usually
unchanged, except beneath and about the invaders, where either adaptation
or extermination results. Finally, it should be borne in mind that the
invasion of a particular formation, especially in the case of layered thickets
and forests, often takes place at two levels, at the height of the facies and
below the facies.
INVESTIGATION OF INVASION
287. The methods to be used in the study of invasion are those already
described elsewhere. The migration circle is of the first importance because
it makes it possible to secure an accurate record of actual movement.
Quadrat and transect are valuable, but from their nature they are more
serviceable for ecesis than for migration. All of these should be of the
permanent type, in order that the fate of invaders may be followed for
several years at least. Permanent areas furnish evidence of the changes
wrought in the actual vegetation, while denuded ones can serve only to
show the potential migration and ecesis of the constituent species.
Transition zones and areas are special seats of invasion; they are best
studied by means of the belt transect and the ecotone chart. The movement
of a line of invaders or of scattered outposts is traced by the use of labeled
stakes at the points concerned. It is clear that this method will yield
conclusive data in regard to the great invasions between regions, such as the
movement of species guilds, the advance of the forest frontier, etc. When
invasion is scattered, factor instruments can not be used to advantage, but
where the invading line is well marked, or where extra-formational areas
occur, a knowledge of the physical factors is a great aid.
Page 309
An invasion that has been completed can not be studied in the manner
indicated. A method of comparison must be used, in order to determine the
original home of the invaders. For this an exact knowledge of the
contiguous formations and of the abundance of the species common to all is
a prerequisite. With this as a basis, it is usually a simple task to refer all the
species of the formation concerned to their proper place in the groups,
indigenous, derived, and adventitious.
indicated. A method of comparison must be used, in order to determine the
original home of the invaders. For this an exact knowledge of the
contiguous formations and of the abundance of the species common to all is
a prerequisite. With this as a basis, it is usually a simple task to refer all the
species of the formation concerned to their proper place in the groups,
indigenous, derived, and adventitious.
Page 310
SUCCESSION
288. Concept. Succession is the phenomenon in which a series of
invasions occurs in the same spot. It is important, however, to distinguish
clearly between succession and invasion, for, while the one is the direct
result of the other, not all invasion produces succession. The number of
invaders must be large enough, or their effect must be sufficiently
modifying or controlling to bring about the gradual decrease or
disappearance of the original occupants, or a succession will not be
established. Partial or temporary invasion can never initiate a succession
unless the reaction of the invaders upon the habitat is very great. Complete
and permanent invasion, on the other hand, regularly produces successions,
except in the rare cases where a stable formation entirely replaces a less
stable one without the intervention of other stages. Succession depends in
the first degree upon invasion in such quantity and of such character that the
reaction of the invaders upon the habitat will prepare the way for further
invasion. The characteristic presence of stages in a succession, which
normally correspond to formations, is due to the peculiar operation of
invasion with reaction. In the case of a denuded habitat, for example,
migration from adjacent formations is constantly taking place, but only a
small number of migrants, especially adapted to somewhat extreme
conditions, are able to become established in it. These reach a maximum
development in size or number, and in so doing react upon the habitat in
such a way that more and more of the dormant disseminules present, as well
as those constantly coming into it, find the conditions favorable for
germination and growth. The latter, as they in turn attain their maximum,
cause the gradual disappearance of the species of the first stage, and at the
same time prepare the way for the individuals of the succeeding formation.
It is at present impossible to determine to what degree this substitution is
due to the struggle for existence between the individuals of each species
and between the somewhat similar species of each stage, and to what degree
it arises out of the physical reaction.
It is evident that geological succession is but a larger expression of the
same phenomenon, dealing with infinitely greater periods of time, and
produced by physical changes of such intensity as to give each geological
288. Concept. Succession is the phenomenon in which a series of
invasions occurs in the same spot. It is important, however, to distinguish
clearly between succession and invasion, for, while the one is the direct
result of the other, not all invasion produces succession. The number of
invaders must be large enough, or their effect must be sufficiently
modifying or controlling to bring about the gradual decrease or
disappearance of the original occupants, or a succession will not be
established. Partial or temporary invasion can never initiate a succession
unless the reaction of the invaders upon the habitat is very great. Complete
and permanent invasion, on the other hand, regularly produces successions,
except in the rare cases where a stable formation entirely replaces a less
stable one without the intervention of other stages. Succession depends in
the first degree upon invasion in such quantity and of such character that the
reaction of the invaders upon the habitat will prepare the way for further
invasion. The characteristic presence of stages in a succession, which
normally correspond to formations, is due to the peculiar operation of
invasion with reaction. In the case of a denuded habitat, for example,
migration from adjacent formations is constantly taking place, but only a
small number of migrants, especially adapted to somewhat extreme
conditions, are able to become established in it. These reach a maximum
development in size or number, and in so doing react upon the habitat in
such a way that more and more of the dormant disseminules present, as well
as those constantly coming into it, find the conditions favorable for
germination and growth. The latter, as they in turn attain their maximum,
cause the gradual disappearance of the species of the first stage, and at the
same time prepare the way for the individuals of the succeeding formation.
It is at present impossible to determine to what degree this substitution is
due to the struggle for existence between the individuals of each species
and between the somewhat similar species of each stage, and to what degree
it arises out of the physical reaction.
It is evident that geological succession is but a larger expression of the
same phenomenon, dealing with infinitely greater periods of time, and
produced by physical changes of such intensity as to give each geological
Page 311
stage its peculiar stamp. If, however, the geological record were sufficiently
complete, we should find unquestionably that these great successions
merely represent the stable termini of many series of smaller changes, such
as are found everywhere in recent or existing vegetation.
289. Kinds of succession. The fundamental causes of succession are
invasion and reaction, but the initial causes of a particular succession are to
be sought in the physical or biological disturbances of a habitat or
formation. With reference to the initial cause, we may distinguish normal
succession, which begins with nudation, and ends in stabilization, and
anomalous succession, in which the facies of an ultimate stage of a normal
succession are replaced by other species, or in which the direction of
movement is radically changed. The former is of universal occurrence and
recurrence; the latter operates upon relatively few ultimate formations. In
the origin of normal successions, nudation may be brought about by the
production of new soils or habitats, or by the destruction of the formation
which already occupies a habitat. In a few cases, the way in which the
habitat arises or becomes denuded is not decisive as to the vegetation that is
developed upon it, but as a rule the cause of nudation plays as important a
part in the development of a succession as does the reaction exerted by the
invaders. The importance of this fact has been insisted upon under invasion.
New soils present extreme conditions for ecesis, possess few or no dormant
disseminules, and in consequence their successions take place slowly and
exhibit many stages. Denuded soils as a rule offer optimum conditions for
ecesis as a result of the action of the previous succession, dormant seeds
and propagules are abundant, and the revegetation of such habitats takes
place rapidly and shows few stages. The former may be termed primary
succession, the latter secondary succession.
PRIMARY SUCCESSIONS
290. These arise on newly formed soils, or upon surfaces exposed for the
first time, which have in consequence never borne vegetation before. In
general they are characteristic of mountain regions, where weathering is the
rule, and of lowlands and shores, where sedimentation or elevation
constantly occur. The principal physical phenomena which bring about the
complete, we should find unquestionably that these great successions
merely represent the stable termini of many series of smaller changes, such
as are found everywhere in recent or existing vegetation.
289. Kinds of succession. The fundamental causes of succession are
invasion and reaction, but the initial causes of a particular succession are to
be sought in the physical or biological disturbances of a habitat or
formation. With reference to the initial cause, we may distinguish normal
succession, which begins with nudation, and ends in stabilization, and
anomalous succession, in which the facies of an ultimate stage of a normal
succession are replaced by other species, or in which the direction of
movement is radically changed. The former is of universal occurrence and
recurrence; the latter operates upon relatively few ultimate formations. In
the origin of normal successions, nudation may be brought about by the
production of new soils or habitats, or by the destruction of the formation
which already occupies a habitat. In a few cases, the way in which the
habitat arises or becomes denuded is not decisive as to the vegetation that is
developed upon it, but as a rule the cause of nudation plays as important a
part in the development of a succession as does the reaction exerted by the
invaders. The importance of this fact has been insisted upon under invasion.
New soils present extreme conditions for ecesis, possess few or no dormant
disseminules, and in consequence their successions take place slowly and
exhibit many stages. Denuded soils as a rule offer optimum conditions for
ecesis as a result of the action of the previous succession, dormant seeds
and propagules are abundant, and the revegetation of such habitats takes
place rapidly and shows few stages. The former may be termed primary
succession, the latter secondary succession.
PRIMARY SUCCESSIONS
290. These arise on newly formed soils, or upon surfaces exposed for the
first time, which have in consequence never borne vegetation before. In
general they are characteristic of mountain regions, where weathering is the
rule, and of lowlands and shores, where sedimentation or elevation
constantly occur. The principal physical phenomena which bring about the
Page 312
formation of new soils are: (1) elevation, (2) volcanic action, (3)
weathering, with or without transport.
291. Succession through elevation. Elevation was of very frequent
occurrence during the earlier, more plastic conditions of the earth, and the
successions arising as a result of it must have been important features of the
vegetation of geological periods. To-day, elevation is of much less
importance in changing physiography, and its operation is confined to
volcanic islands, coral reefs, and islets, and to rare movements or
displacements in seacoasts, lake beds, shore lines, etc. There has been no
investigation of the development of vegetation on islands that are rising, or
have recently been elevated, probably because of the slow growth of coral
reefs and the rare appearance of volcanic islands. On coral reefs, the first
vegetation is invariably marine, but as the reef rises higher above the surf
line and the tide, the vegetation passes into a xerophytic terrestrial type
adapted to an impervious rock soil, and ultimately becomes mesophytic. In
volcanic islands, unless they are mere rocks over which the waves rush, the
succession must always begin with a xerophytic rock formation. The best
known example of a rising coast line is found in Norway and Sweden,
where the southeastern coast is rising at the rate of five or six feet a century.
There can be little question that such changes of level will produce marked
changes in vegetation, but the modification will be so gradual as to be
scarcely perceptible in a single generation. It is probable that the forests of
the Atlantic coastal plains are the ultimate stages of successions initiated at
the time of the final elevation of the sea bottom along the coast line.
weathering, with or without transport.
291. Succession through elevation. Elevation was of very frequent
occurrence during the earlier, more plastic conditions of the earth, and the
successions arising as a result of it must have been important features of the
vegetation of geological periods. To-day, elevation is of much less
importance in changing physiography, and its operation is confined to
volcanic islands, coral reefs, and islets, and to rare movements or
displacements in seacoasts, lake beds, shore lines, etc. There has been no
investigation of the development of vegetation on islands that are rising, or
have recently been elevated, probably because of the slow growth of coral
reefs and the rare appearance of volcanic islands. On coral reefs, the first
vegetation is invariably marine, but as the reef rises higher above the surf
line and the tide, the vegetation passes into a xerophytic terrestrial type
adapted to an impervious rock soil, and ultimately becomes mesophytic. In
volcanic islands, unless they are mere rocks over which the waves rush, the
succession must always begin with a xerophytic rock formation. The best
known example of a rising coast line is found in Norway and Sweden,
where the southeastern coast is rising at the rate of five or six feet a century.
There can be little question that such changes of level will produce marked
changes in vegetation, but the modification will be so gradual as to be
scarcely perceptible in a single generation. It is probable that the forests of
the Atlantic coastal plains are the ultimate stages of successions initiated at
the time of the final elevation of the sea bottom along the coast line.
Page 313
Fig. 60. A lichen formation (Lecanora-Physcia-petrium), the first stage of the typical primary
succession (Lecanora-Picea-sphyrium) of the Colorado mountains.
292. Succession through volcanic action. The deposition of volcanic
ashes and flows of lava are relatively infrequent at present, occurring only
in the immediate vicinity of active volcanoes, chiefly in or near the tropics.
Successions of this sort are in consequence not only rare, but they are also
relatively inaccessible to investigators. They have been studied in a few
cases, for example, those of Krakatoa by Treub, but this study has been
confined to the general features of revegetation. Ash fields and lava beds
are widely different in compactness, but they agree in having a low water-
and nutrition-content. The pioneer plants in both will be intense xerophytes,
but the soil differences will determine that these shall be sand-binders in the
former, and rock-weathering plants in the latter.
293. Weathering. Practically all primary successions start on soils
produced by weathering. This is also true of coral or volcanic islets and of
succession (Lecanora-Picea-sphyrium) of the Colorado mountains.
292. Succession through volcanic action. The deposition of volcanic
ashes and flows of lava are relatively infrequent at present, occurring only
in the immediate vicinity of active volcanoes, chiefly in or near the tropics.
Successions of this sort are in consequence not only rare, but they are also
relatively inaccessible to investigators. They have been studied in a few
cases, for example, those of Krakatoa by Treub, but this study has been
confined to the general features of revegetation. Ash fields and lava beds
are widely different in compactness, but they agree in having a low water-
and nutrition-content. The pioneer plants in both will be intense xerophytes,
but the soil differences will determine that these shall be sand-binders in the
former, and rock-weathering plants in the latter.
293. Weathering. Practically all primary successions start on soils
produced by weathering. This is also true of coral or volcanic islets and of
Page 314
lava beds, for no terrestrial vegetation can secure a foothold upon them until
the surface of the rock has been to some extent decomposed or
disintegrated. Weathering, as is well known, consists of two processes,
disintegration and decomposition, which usually operate successively,
though they are sometimes concomitant. Disintegration usually precedes,
especially in rock masses, and unless it is soon followed by decomposition,
results in dysgeogenous soils. Decomposition often goes hand in hand with
disintegration, or it takes place so rapidly and perfectly that it alone seems
to be present. In either case, the resulting soil is eugeogenous. The relation
of decomposition to disintegration determines the size and compactness of
the soil particles, and upon the latter depend the porosity, capillarity, and
hygroscopicity of the soil. These control in large degree the character of the
first vegetation to appear on the soil.
Another point of fundamental value in determining revegetation is the
disposition of the weathered rock. If it remains in situ, it will evidently
differ in respect to compactness, homogeneity, nutrition-content, water-
content, disseminules, etc., from weathered material which has been
transported. An essential difference also arises from the fact that a rock may
be weathered a long distance from the place where the decomposed
particles are finally deposited, and in the midst of a vegetation very
different from that found in the region of deposit. The disposition of the
weathered material affords in consequence a satisfactory basis for the
arrangement of primary successions. The following classification is
proposed, based upon the soil groups established by Merrill.[39]
294. Succession in residuary soils. Residuary soils are always sedentary,
i. e., they are formed in situ. They show certain differences dependent upon
the rock from which they originate, which may be mixed crystalline shale,
sandstone, or limestone, but the thoroughness of decomposition causes
these differences to be comparatively small. Residuary soils are typically
eugeogenous; their successions in consequence usually begin with
mesophytes, and consist of a few stages. The soluble salt-content is
comparatively low, since all soluble matters are readily leached out.
Successions in these soils are especially characteristic of shale, sandstone,
and limestone ledges or banks. Cumulose deposits, like residuary ones, are
sedentary in character, but as they are produced by the accumulation of
organic matter, they will be considered under reactions of vegetation upon
habitat.
the surface of the rock has been to some extent decomposed or
disintegrated. Weathering, as is well known, consists of two processes,
disintegration and decomposition, which usually operate successively,
though they are sometimes concomitant. Disintegration usually precedes,
especially in rock masses, and unless it is soon followed by decomposition,
results in dysgeogenous soils. Decomposition often goes hand in hand with
disintegration, or it takes place so rapidly and perfectly that it alone seems
to be present. In either case, the resulting soil is eugeogenous. The relation
of decomposition to disintegration determines the size and compactness of
the soil particles, and upon the latter depend the porosity, capillarity, and
hygroscopicity of the soil. These control in large degree the character of the
first vegetation to appear on the soil.
Another point of fundamental value in determining revegetation is the
disposition of the weathered rock. If it remains in situ, it will evidently
differ in respect to compactness, homogeneity, nutrition-content, water-
content, disseminules, etc., from weathered material which has been
transported. An essential difference also arises from the fact that a rock may
be weathered a long distance from the place where the decomposed
particles are finally deposited, and in the midst of a vegetation very
different from that found in the region of deposit. The disposition of the
weathered material affords in consequence a satisfactory basis for the
arrangement of primary successions. The following classification is
proposed, based upon the soil groups established by Merrill.[39]
294. Succession in residuary soils. Residuary soils are always sedentary,
i. e., they are formed in situ. They show certain differences dependent upon
the rock from which they originate, which may be mixed crystalline shale,
sandstone, or limestone, but the thoroughness of decomposition causes
these differences to be comparatively small. Residuary soils are typically
eugeogenous; their successions in consequence usually begin with
mesophytes, and consist of a few stages. The soluble salt-content is
comparatively low, since all soluble matters are readily leached out.
Successions in these soils are especially characteristic of shale, sandstone,
and limestone ledges or banks. Cumulose deposits, like residuary ones, are
sedentary in character, but as they are produced by the accumulation of
organic matter, they will be considered under reactions of vegetation upon
habitat.
Page 315
Fig. 61. Talus arising from the disintegration of a granitic cliff; the rocks are covered with crustose
lichens.
295. Succession in colluvial soils. Colluvial deposits owe their
aggregation solely or chiefly to the action of gravity. They are the
immediate result of the disintegration of cliffs, ledges, and mountain sides,
decomposition appearing later as a secondary factor. The masses and
particles arising from disintegration are extremely variable in size, but they
agree as a rule in their angular shape. The typical example of the colluvial
deposit is the talus, which may originate from any kind of rock, and
contains pieces of all sizes. Gravel slides differ from ordinary talus in being
composed of more uniform particles, which are worn round by slipping
down the slope in response to gravity and surface wash. Boulder fields are
to be regarded as talus produced by weathering under the influence of
joints, resulting in huge boulders which become more and more rounded
under the action of water and gravity. This statement applies to those fields
lichens.
295. Succession in colluvial soils. Colluvial deposits owe their
aggregation solely or chiefly to the action of gravity. They are the
immediate result of the disintegration of cliffs, ledges, and mountain sides,
decomposition appearing later as a secondary factor. The masses and
particles arising from disintegration are extremely variable in size, but they
agree as a rule in their angular shape. The typical example of the colluvial
deposit is the talus, which may originate from any kind of rock, and
contains pieces of all sizes. Gravel slides differ from ordinary talus in being
composed of more uniform particles, which are worn round by slipping
down the slope in response to gravity and surface wash. Boulder fields are
to be regarded as talus produced by weathering under the influence of
joints, resulting in huge boulders which become more and more rounded
under the action of water and gravity. This statement applies to those fields
Page 316
which are in connection with some cliff that is weathering in this fashion;
otherwise, boulder fields are of aqueous or glacial origin. The character of
the successions in talus will depend upon the kind of rock in the latter. If the
rock is igneous or metamorphic, decomposition will be slow, and the soil
will be dysgeogenous. Successions on such talus consist of many stages,
and the formations are for a long time open and xerophytic. In talus formed
from sedimentary rocks, especially shales, limestones, and calcareous
sandstones, decomposition is much more rapid, and the successions are
simpler and more mesophytic.
296. Succession in alluvial soils. Alluvial soils are fluvial when laid
down by streams and rivers, and litoral when washed up by the waves or
tides. They are formed when any obstacle retards the movement of the
water, decreasing its carrying power, and causing the deposit of part or all
of its load. They consist of more or less rounded, finely comminuted
particles, mingled with organic matter and detritus. Alluvial deposits are
especially frequent at the mouth of streams and rivers, on their terraces and
flood plains, and along silting banks as compared with the erosion banks of
meanders. The filling of ponds by the erosion due to surface drainage, and
of lakes by the deposition of the loads of streams that enter them, results in
the formation of new alluvium. A similar phenomenon occurs along coasts,
where bays and inlets are slowly converted into marshes in consequence of
being shallowed by the material washed in by the waves and tides. Such
paludal deposits are invariably salt water or brackish. Contrasted with these,
which are uniformly black in consequence of the large amount of organic
matter present, are the sandbars and beaches, which, though due to the same
agents, are light grey or white in color, because of the constant leaching by
the waves. Two kinds of alluvial deposits may accordingly be distinguished:
(1) those black with organic matter, and little disturbed by water, and (2)
those of a light color, which are constantly swept by the waves. The
successions corresponding to these are radically different. In the first, the
pioneer vegetation is hydrophytic, consisting largely of amphibious plants.
The pioneer stages retard the movement of the water more and more, and
correspondingly hasten the deposition of its load. The marsh bed slowly
rises in consequence, and finally the marsh begins to dry out, passing first
into a wet meadow, and then into a meadow of the normal type. A notable
exception to this sequence occurs when the swamp contains organic matter
or salts in excess, in which case the vegetation consists indefinitely of
otherwise, boulder fields are of aqueous or glacial origin. The character of
the successions in talus will depend upon the kind of rock in the latter. If the
rock is igneous or metamorphic, decomposition will be slow, and the soil
will be dysgeogenous. Successions on such talus consist of many stages,
and the formations are for a long time open and xerophytic. In talus formed
from sedimentary rocks, especially shales, limestones, and calcareous
sandstones, decomposition is much more rapid, and the successions are
simpler and more mesophytic.
296. Succession in alluvial soils. Alluvial soils are fluvial when laid
down by streams and rivers, and litoral when washed up by the waves or
tides. They are formed when any obstacle retards the movement of the
water, decreasing its carrying power, and causing the deposit of part or all
of its load. They consist of more or less rounded, finely comminuted
particles, mingled with organic matter and detritus. Alluvial deposits are
especially frequent at the mouth of streams and rivers, on their terraces and
flood plains, and along silting banks as compared with the erosion banks of
meanders. The filling of ponds by the erosion due to surface drainage, and
of lakes by the deposition of the loads of streams that enter them, results in
the formation of new alluvium. A similar phenomenon occurs along coasts,
where bays and inlets are slowly converted into marshes in consequence of
being shallowed by the material washed in by the waves and tides. Such
paludal deposits are invariably salt water or brackish. Contrasted with these,
which are uniformly black in consequence of the large amount of organic
matter present, are the sandbars and beaches, which, though due to the same
agents, are light grey or white in color, because of the constant leaching by
the waves. Two kinds of alluvial deposits may accordingly be distinguished:
(1) those black with organic matter, and little disturbed by water, and (2)
those of a light color, which are constantly swept by the waves. The
successions corresponding to these are radically different. In the first, the
pioneer vegetation is hydrophytic, consisting largely of amphibious plants.
The pioneer stages retard the movement of the water more and more, and
correspondingly hasten the deposition of its load. The marsh bed slowly
rises in consequence, and finally the marsh begins to dry out, passing first
into a wet meadow, and then into a meadow of the normal type. A notable
exception to this sequence occurs when the swamp contains organic matter
or salts in excess, in which case the vegetation consists indefinitely of
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swamp xerophytes, or halophytes. The first vegetation on fresh water
sandbars is xerophytic, or, properly, dissophytic, unless they remain water-
swept, and the ultimate stages of their successions are mesophytic
woodlands composed of water-loving genera, Populus, Salix, etc. It seems
certain, however, that these will finally give way to longer-lived hardwoods.
Maritime sandbars and beaches are always saline, and their successions run
their short course of development entirely within the group of halophytes,
unless the retreat of the sea or freshwater floods change the character of the
soil. The chemical action of underground waters also produces new soils,
which might be classed as alluvial. These soils are essentially rock deposits,
travertine, silicious sinter, etc., made by iron and lime springs and by
geysers, and they must be changed by decomposition into soils proper to be
comparable with alluvial soils.
sandbars is xerophytic, or, properly, dissophytic, unless they remain water-
swept, and the ultimate stages of their successions are mesophytic
woodlands composed of water-loving genera, Populus, Salix, etc. It seems
certain, however, that these will finally give way to longer-lived hardwoods.
Maritime sandbars and beaches are always saline, and their successions run
their short course of development entirely within the group of halophytes,
unless the retreat of the sea or freshwater floods change the character of the
soil. The chemical action of underground waters also produces new soils,
which might be classed as alluvial. These soils are essentially rock deposits,
travertine, silicious sinter, etc., made by iron and lime springs and by
geysers, and they must be changed by decomposition into soils proper to be
comparable with alluvial soils.
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Fig. 62. Talus arising from the decomposition of granite; the gravel is covered with a formation of
foliose lichens (Parmelia-chalicium), the second stage of the primary talus succession; the herbs are
pioneers of the next stage.
297. Succession in aeolian soils. The only wind-borne soils of
geological importance at the present time are those which form dunes, both
inland and coastal. Aeolian deposits consist largely of rounded sand
particles, which are of almost uniform size in any particular dune, but vary
greatly in dunes of different ages. The reaction of the pioneers on dunes
plays an important part in building the latter, but the immense dunes of
inland deserts, which are entirely destitute of vegetation, seem to indicate
that its value has been overestimated. The first stages in dune successions
are dissophytic, i. e., the plants grow in a soil of medium or high water-
content, but in an atmosphere that is extremely xerophytic. The ultimate
stages vary widely in accordance with the region in which they occur; they
may be xerophytic heaths or mesophytic meadows and forests. Because of
foliose lichens (Parmelia-chalicium), the second stage of the primary talus succession; the herbs are
pioneers of the next stage.
297. Succession in aeolian soils. The only wind-borne soils of
geological importance at the present time are those which form dunes, both
inland and coastal. Aeolian deposits consist largely of rounded sand
particles, which are of almost uniform size in any particular dune, but vary
greatly in dunes of different ages. The reaction of the pioneers on dunes
plays an important part in building the latter, but the immense dunes of
inland deserts, which are entirely destitute of vegetation, seem to indicate
that its value has been overestimated. The first stages in dune successions
are dissophytic, i. e., the plants grow in a soil of medium or high water-
content, but in an atmosphere that is extremely xerophytic. The ultimate
stages vary widely in accordance with the region in which they occur; they
may be xerophytic heaths or mesophytic meadows and forests. Because of
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their striking character and economic significance, dunes have received
much attention, with the result that their successions are the most
thoroughly known of all. Prairie and steppe formations are probably to be
regarded as the ultimate stages of successions established on wind-borne
loess, and it is possible that the same is true of sand-hill vegetation in the
prairie province.
298. Succession in glacial soils. The formation of glacial deposits is at
present confined to alpine and arctic regions. Recent successions in such
soils are localized in these regions, and are in consequence relatively
unimportant. There can be little question, however, that the thorough
investigation of succession in and near the moraines of existing glaciers will
throw much light upon the successions of the glacial period. Moraines,
drumlins, eskars, and alluvial cones represent the various kinds of glacial
deposits. They agree in being heterogeneous in composition, and are
covered to-day with ultimate stages of vegetation, except in the immediate
vicinity of glaciers.
SECONDARY SUCCESSIONS
299. Generally speaking, all successions on denuded soils are secondary.
When vegetation is completely removed by excessive erosion, it is an open
question whether the resulting habitat is to be regarded as new or denuded.
Erosion is rarely so extreme and so rapid, however, as to produce such a
condition, even when it results from cultivation or deforestation. It is,
moreover, especially characteristic of newly formed soils, and in studying
succession in eroded habitats, it is fundamentally important to determine
whether erosion has produced denudation, or has operated upon a new soil.
The great majority of secondary successions owe their origin to floods,
animals, or the activities of man, and they agree in occurring upon
decomposed soils of medium water-content, which contain considerable
organic matter, and a large number of dormant migrants. These successions
consist of relatively few stages, and are rarely of extreme character.
300. Succession in eroded soils. Eroded soils show considerable
differences, as they arise in consequence of erosion by water or by wind,
though the initial stages of revegetation derive their character more from the
aggregation of the soil than from the nature of the erosive agent. Eroded
much attention, with the result that their successions are the most
thoroughly known of all. Prairie and steppe formations are probably to be
regarded as the ultimate stages of successions established on wind-borne
loess, and it is possible that the same is true of sand-hill vegetation in the
prairie province.
298. Succession in glacial soils. The formation of glacial deposits is at
present confined to alpine and arctic regions. Recent successions in such
soils are localized in these regions, and are in consequence relatively
unimportant. There can be little question, however, that the thorough
investigation of succession in and near the moraines of existing glaciers will
throw much light upon the successions of the glacial period. Moraines,
drumlins, eskars, and alluvial cones represent the various kinds of glacial
deposits. They agree in being heterogeneous in composition, and are
covered to-day with ultimate stages of vegetation, except in the immediate
vicinity of glaciers.
SECONDARY SUCCESSIONS
299. Generally speaking, all successions on denuded soils are secondary.
When vegetation is completely removed by excessive erosion, it is an open
question whether the resulting habitat is to be regarded as new or denuded.
Erosion is rarely so extreme and so rapid, however, as to produce such a
condition, even when it results from cultivation or deforestation. It is,
moreover, especially characteristic of newly formed soils, and in studying
succession in eroded habitats, it is fundamentally important to determine
whether erosion has produced denudation, or has operated upon a new soil.
The great majority of secondary successions owe their origin to floods,
animals, or the activities of man, and they agree in occurring upon
decomposed soils of medium water-content, which contain considerable
organic matter, and a large number of dormant migrants. These successions
consist of relatively few stages, and are rarely of extreme character.
300. Succession in eroded soils. Eroded soils show considerable
differences, as they arise in consequence of erosion by water or by wind,
though the initial stages of revegetation derive their character more from the
aggregation of the soil than from the nature of the erosive agent. Eroded
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soils are as a rule xerophytic. In the case of erosion by water, dysgeogenous
soils are readily worn away in consequence of their lack of cohesion, as in
sand draws, etc., while eugeogenous soils are easily eroded only on slopes,
as in the case of ravines, hillsides, etc. In the former, the extreme porosity
and slight capillarity of the sand and gravel result in a low water-content. In
the finer soils, the water-content is also low, on account of the excessive
run-off, due to compactness of the particles and to the slope. The erosive
action of winds upon soils bearing vegetation is not very general; it is found
to some extent in more or less established dunes, and exists in a marked
degree in buttes, mushroom rocks, and blowouts. The first two are regularly
xerophytic, the last as a rule, dissophytic. The early stages of successions in
eroded soils are composed of xerophytes. In loose soils, these are forms
capable of binding the soil particles together, thus preventing wash, and
increasing the accumulation of fine particles, especially of organic matter.
In compact soils, the effect is much the same; the pioneers not only
decrease erosion, but at the same time also increase the water-content by
retarding the movement of the run-off.
301. Succession in flooded soils. The universal response of vegetation to
floods is found in the amphibious plant, which is a plastic form capable of
adjustment to very different water-contents. Floods are confined largely to
river basins and coasts. In hilly and mountainous regions, where the slope is
great, any considerable accumulation of flood waters is now impossible,
although of frequent occurrence when land forms were more plastic.
In all streams that have become graded, the fall is insufficient to carry off
the surplus water in the spring when snows are melting rapidly, or at times
of unusual precipitation. These waters accumulate, and, overflowing the
banks, spread out over the lowlands, resulting in the formation of a well-
defined flood plain. This is a periodical occurrence with mature streams,
and it occurs more or less regularly with all that are not torrent-like in
character. The effect of the overflow is to destroy or to place at a
disadvantage those plants of the flood plain that are not hydrophytes. At the
same time, a thin layer of fresh silt is deposited upon the valley floor of
sand or alluvium. Flooding is most frequent and of longest duration near the
banks of the stream. It extends more or less uniformly over the flood plain,
and disappears gradually or abruptly as the latter rises into the bench above.
Floods destroy vegetation and make a place for secondary successions by
drowning out mesophytic species, by washing away the aquatic forms of
soils are readily worn away in consequence of their lack of cohesion, as in
sand draws, etc., while eugeogenous soils are easily eroded only on slopes,
as in the case of ravines, hillsides, etc. In the former, the extreme porosity
and slight capillarity of the sand and gravel result in a low water-content. In
the finer soils, the water-content is also low, on account of the excessive
run-off, due to compactness of the particles and to the slope. The erosive
action of winds upon soils bearing vegetation is not very general; it is found
to some extent in more or less established dunes, and exists in a marked
degree in buttes, mushroom rocks, and blowouts. The first two are regularly
xerophytic, the last as a rule, dissophytic. The early stages of successions in
eroded soils are composed of xerophytes. In loose soils, these are forms
capable of binding the soil particles together, thus preventing wash, and
increasing the accumulation of fine particles, especially of organic matter.
In compact soils, the effect is much the same; the pioneers not only
decrease erosion, but at the same time also increase the water-content by
retarding the movement of the run-off.
301. Succession in flooded soils. The universal response of vegetation to
floods is found in the amphibious plant, which is a plastic form capable of
adjustment to very different water-contents. Floods are confined largely to
river basins and coasts. In hilly and mountainous regions, where the slope is
great, any considerable accumulation of flood waters is now impossible,
although of frequent occurrence when land forms were more plastic.
In all streams that have become graded, the fall is insufficient to carry off
the surplus water in the spring when snows are melting rapidly, or at times
of unusual precipitation. These waters accumulate, and, overflowing the
banks, spread out over the lowlands, resulting in the formation of a well-
defined flood plain. This is a periodical occurrence with mature streams,
and it occurs more or less regularly with all that are not torrent-like in
character. The effect of the overflow is to destroy or to place at a
disadvantage those plants of the flood plain that are not hydrophytes. At the
same time, a thin layer of fresh silt is deposited upon the valley floor of
sand or alluvium. Flooding is most frequent and of longest duration near the
banks of the stream. It extends more or less uniformly over the flood plain,
and disappears gradually or abruptly as the latter rises into the bench above.
Floods destroy vegetation and make a place for secondary successions by
drowning out mesophytic species, by washing away the aquatic forms of
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ponds and pools, and by the erosion of banks and sandbars. They affect the
amphibious vegetation of swamp and shore to a certain extent, but, unless
the period of flooding is long, they tend to emphasize such formations
rather than to destroy them. The still-water formations of many cutoff and
oxbow lakes owe their origin to a river which cuts across a meander in time
of flood. This result is more often attained by the alternate silting and
erosion of a meandering river by which it cuts across a bend in its channel.
The usual successions in flooded lands are short as a rule; amphibious
algae, liverworts, and mosses soon give way to ruderal plants, and these in
turn to the original mesophytes of meadows, or dissophytes of sandbars. In
the case of ponds and pools, the process of washing-out or silting up merely
removes or destroys the vegetation, without effectively modifying the
habitat, and the secondary successions that follow are extremely short.
302. Succession by subsidence. Subsidence is a factor of the most
profound importance in changing vegetation. It operates over vast areas
through immense periods of time. For these reasons, the changes are so
slow as to be almost imperceptible, and the resulting successions can be
studied only in the geological record. Extensive subsidence is confined to-
day to coastal plains, as in Greenland, the south Atlantic coast, and the
region of the Mississippi delta, where its effects are merged with the
paludation of tidal rivers, and the wave and tide erosion of the sea shore.
Such successions are unique, inasmuch as the denuding force operates very
slowly instead of quickly, and the first pioneers of the new vegetation
appear before the original formation has been destroyed. In all cases, the
succession is from mesophytic or halophytic formations to paludose, and,
finally, marine vegetation. In small areas of subsidence, such as shore slips
along lakes and streams, sink holes, and sunken bogs, the succession is
usually both short and simple, mesophytes giving place to amphibious and
ultimately to aquatic forms.
303. Successions in landslips. Landslips occur only in montane and hilly
regions, and here they are merely of local importance. In many respects,
they are not unlike talus; they show essential differences, however, in that
they are not sorted by gravity, and in that they destroy vegetation almost
instantly. The succession arises as a rule, not upon the original soil, but
upon that of the landslip, and, as pointed out elsewhere, might well be
regarded as primary.
amphibious vegetation of swamp and shore to a certain extent, but, unless
the period of flooding is long, they tend to emphasize such formations
rather than to destroy them. The still-water formations of many cutoff and
oxbow lakes owe their origin to a river which cuts across a meander in time
of flood. This result is more often attained by the alternate silting and
erosion of a meandering river by which it cuts across a bend in its channel.
The usual successions in flooded lands are short as a rule; amphibious
algae, liverworts, and mosses soon give way to ruderal plants, and these in
turn to the original mesophytes of meadows, or dissophytes of sandbars. In
the case of ponds and pools, the process of washing-out or silting up merely
removes or destroys the vegetation, without effectively modifying the
habitat, and the secondary successions that follow are extremely short.
302. Succession by subsidence. Subsidence is a factor of the most
profound importance in changing vegetation. It operates over vast areas
through immense periods of time. For these reasons, the changes are so
slow as to be almost imperceptible, and the resulting successions can be
studied only in the geological record. Extensive subsidence is confined to-
day to coastal plains, as in Greenland, the south Atlantic coast, and the
region of the Mississippi delta, where its effects are merged with the
paludation of tidal rivers, and the wave and tide erosion of the sea shore.
Such successions are unique, inasmuch as the denuding force operates very
slowly instead of quickly, and the first pioneers of the new vegetation
appear before the original formation has been destroyed. In all cases, the
succession is from mesophytic or halophytic formations to paludose, and,
finally, marine vegetation. In small areas of subsidence, such as shore slips
along lakes and streams, sink holes, and sunken bogs, the succession is
usually both short and simple, mesophytes giving place to amphibious and
ultimately to aquatic forms.
303. Successions in landslips. Landslips occur only in montane and hilly
regions, and here they are merely of local importance. In many respects,
they are not unlike talus; they show essential differences, however, in that
they are not sorted by gravity, and in that they destroy vegetation almost
instantly. The succession arises as a rule, not upon the original soil, but
upon that of the landslip, and, as pointed out elsewhere, might well be
regarded as primary.
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304. Succession in drained, or dried soils. In geological times, the
subsidence of barriers must often have produced drainage and drying-out,
just as elevation frequently resulted in flooding and lake formation. At the
present time, the drying-out of lakes and ponds is the result of artificial
drainage, or of climatic changes. The former will be considered under
successions brought about by the agency of man. Climatic changes when
general operate so slowly that the stages of such successions are perceptible
only when recorded in strata. More locally, climate swings back and forth
through a period of years, with the result that in dry years the swamps and
ponds of wetter seasons are dried out, and the vegetation destroyed or
changed. If the process be gradual, the succession passes from hydrophytic
through amphibious to mesophytic, and, in dry regions, xerophytic
conditions. When the process of drying-out occurs rapidly, as in a single
summer, the original formation is destroyed, and the new vegetation
consists largely of ruderal plants. A peculiar effect of climate occurs in
regions with poor drainage, where the result of intense evaporation is to
produce alkaline basins and salt lakes, in which the succession becomes
more and more open, and is finally represented by a few stabilized
halophytes, or disappears completely.
subsidence of barriers must often have produced drainage and drying-out,
just as elevation frequently resulted in flooding and lake formation. At the
present time, the drying-out of lakes and ponds is the result of artificial
drainage, or of climatic changes. The former will be considered under
successions brought about by the agency of man. Climatic changes when
general operate so slowly that the stages of such successions are perceptible
only when recorded in strata. More locally, climate swings back and forth
through a period of years, with the result that in dry years the swamps and
ponds of wetter seasons are dried out, and the vegetation destroyed or
changed. If the process be gradual, the succession passes from hydrophytic
through amphibious to mesophytic, and, in dry regions, xerophytic
conditions. When the process of drying-out occurs rapidly, as in a single
summer, the original formation is destroyed, and the new vegetation
consists largely of ruderal plants. A peculiar effect of climate occurs in
regions with poor drainage, where the result of intense evaporation is to
produce alkaline basins and salt lakes, in which the succession becomes
more and more open, and is finally represented by a few stabilized
halophytes, or disappears completely.
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Fig. 63. A typical gravel slide (talus) of the Rocky mountains, before invasion.
305. Succession by animal agency. Successions of this class are
altogether of secondary importance, the instances in which animals produce
denudation being relatively few. Such are the heaps of dirt thrown up by
prairie dogs and other burrowing animals, upon which ruderal plants are
first established, to be finally crowded out by the species of the original
formation. Buffalo wallows furnish examples of similar successions in
which the initial stages are subruderal, while overstocking and overgrazing
frequently produce the same result with ruderal plants.
306. Succession by human agency. The activities of man in changing
the surface of the earth are so diverse that it is impossible to fit the resulting
successions in a natural system. While man does not exactly make new
soils, he exposes soils in various operations: mining, irrigation, railroad
building, etc. He destroys vegetation by fires, lumbering, cultivation, and
305. Succession by animal agency. Successions of this class are
altogether of secondary importance, the instances in which animals produce
denudation being relatively few. Such are the heaps of dirt thrown up by
prairie dogs and other burrowing animals, upon which ruderal plants are
first established, to be finally crowded out by the species of the original
formation. Buffalo wallows furnish examples of similar successions in
which the initial stages are subruderal, while overstocking and overgrazing
frequently produce the same result with ruderal plants.
306. Succession by human agency. The activities of man in changing
the surface of the earth are so diverse that it is impossible to fit the resulting
successions in a natural system. While man does not exactly make new
soils, he exposes soils in various operations: mining, irrigation, railroad
building, etc. He destroys vegetation by fires, lumbering, cultivation, and
Page 324
drainage, and if he can not control climate, he at least modifies its natural
effects by irrigation and the conservation of moisture. The operations of
man extend from seacoasts and swampy lowlands through mesophytic
forests and prairies to the driest uplands and inlands. Since the adjacent
formations determine in large degree the course and constitution of a
succession, it will be seen that the effects of any particular activity upon
vegetation will differ greatly in different regions. For convenience, all
classes of successions arising from the presence and activity of man will be
considered in this place, though, as indicated above, some might well be
regarded as producing primary successions, while others produce
anomalous ones.
307. Succession in burned areas. It will suffice merely to point out that
“burns” may arise naturally through lightning, volcanic cinders, lava flows,
etc., but the chances are so slight that these causes may be ignored. The
causes of fires are legion, and as they have little or no effect upon results,
they need not be considered. From their nature, fires are of little
significance in open vegetation, deserts, polar barrens, alpine fields, etc.,
since the area of the burn can never be large. In closed formations, the
extent of fires is limited only by the area of the vegetation, and the effect of
wind, rain, and other forces. Forest fires usually occur during the resting
period, except in the case of coniferous forests. In grassland, the living parts
are underground during autumn and winter, when prairie fires commonly
occur. As a consequence, the repeated annual burning of meadow or prairie
does not result in denudation and subsequent succession. On the contrary, it
acts in part as a stabilizing agent, inasmuch as it injures the typical
vegetation forms of grassland much less than it does the woody invaders.
All formations with perennial parts above ground, viz., thicket, open
woodland, and forest, are seriously injured by fire. A severe general fire
destroys the vegetation completely; a local fire destroys the formation in
restricted areas; while a slight or superficial burn removes the undergrowth
and hastens the disappearance of the weaker trees. In the latter case, while
the primary layer of the forest remains the same, succession takes place in
the herbaceous and shrubby layers. These successions are peculiar in that
they are composed almost wholly of the proper species of the forest, and
that they are very short, showing only a few poorly defined stages. A local
fire initiates a succession in which the pioneers are derived largely from the
original formation, particularly when the latter encloses the burned area
effects by irrigation and the conservation of moisture. The operations of
man extend from seacoasts and swampy lowlands through mesophytic
forests and prairies to the driest uplands and inlands. Since the adjacent
formations determine in large degree the course and constitution of a
succession, it will be seen that the effects of any particular activity upon
vegetation will differ greatly in different regions. For convenience, all
classes of successions arising from the presence and activity of man will be
considered in this place, though, as indicated above, some might well be
regarded as producing primary successions, while others produce
anomalous ones.
307. Succession in burned areas. It will suffice merely to point out that
“burns” may arise naturally through lightning, volcanic cinders, lava flows,
etc., but the chances are so slight that these causes may be ignored. The
causes of fires are legion, and as they have little or no effect upon results,
they need not be considered. From their nature, fires are of little
significance in open vegetation, deserts, polar barrens, alpine fields, etc.,
since the area of the burn can never be large. In closed formations, the
extent of fires is limited only by the area of the vegetation, and the effect of
wind, rain, and other forces. Forest fires usually occur during the resting
period, except in the case of coniferous forests. In grassland, the living parts
are underground during autumn and winter, when prairie fires commonly
occur. As a consequence, the repeated annual burning of meadow or prairie
does not result in denudation and subsequent succession. On the contrary, it
acts in part as a stabilizing agent, inasmuch as it injures the typical
vegetation forms of grassland much less than it does the woody invaders.
All formations with perennial parts above ground, viz., thicket, open
woodland, and forest, are seriously injured by fire. A severe general fire
destroys the vegetation completely; a local fire destroys the formation in
restricted areas; while a slight or superficial burn removes the undergrowth
and hastens the disappearance of the weaker trees. In the latter case, while
the primary layer of the forest remains the same, succession takes place in
the herbaceous and shrubby layers. These successions are peculiar in that
they are composed almost wholly of the proper species of the forest, and
that they are very short, showing only a few poorly defined stages. A local
fire initiates a succession in which the pioneers are derived largely from the
original formation, particularly when the latter encloses the burned area
Page 325
more or less completely. The constitution of the intermediate and ultimate
stages will depend in a larger degree still upon the size and position of the
burn. When a particular formation is destroyed wholly or in large part, the
first stages of the new vegetation are made up by invaders from the adjacent
formations. In the most perfect types of succession, this dissimilarity
between the new and the old vegetation continues to the last stage, in which
the reappearance of the facies precedes that of the subordinate layers. In
many forest successions, however, the general physical similarity of the
ultimate stages permits the early reappearance of the herbaceous and
shrubby species, and the final stages affect the facies alone. Successions in
burned areas operate usually within the water-content groups. The
reconstruction of a mesophytic forest takes place by means of mesophytes;
of the rarer xerophytic and hydrophytic forests, through xerophytes and
hydrophytes respectively. This is due to the fact that the alteration of the
soil is slight, except where the burning of the vegetation permits the
entrance of erosion, as on mountain slopes.
stages will depend in a larger degree still upon the size and position of the
burn. When a particular formation is destroyed wholly or in large part, the
first stages of the new vegetation are made up by invaders from the adjacent
formations. In the most perfect types of succession, this dissimilarity
between the new and the old vegetation continues to the last stage, in which
the reappearance of the facies precedes that of the subordinate layers. In
many forest successions, however, the general physical similarity of the
ultimate stages permits the early reappearance of the herbaceous and
shrubby species, and the final stages affect the facies alone. Successions in
burned areas operate usually within the water-content groups. The
reconstruction of a mesophytic forest takes place by means of mesophytes;
of the rarer xerophytic and hydrophytic forests, through xerophytes and
hydrophytes respectively. This is due to the fact that the alteration of the
soil is slight, except where the burning of the vegetation permits the
entrance of erosion, as on mountain slopes.
Page 326
Fig. 64. Gravel slide formation (Pseudocymopterus-Mentzelia-chalicium), stage III of the talus
succession.
308. Succession in lumbered areas. Commercial lumbering, especially
where practiced for wood-pulp as well as for timber, results in complete or
nearly complete destruction of the vegetation by removal and the change
from diffuse light to sunlight, or by the action of erosion upon the exposed
surface. In the first place, short mesophytic successions will result; in the
second, the successions will be long and complex, passing through
decreasingly xerophytic conditions to a stable mesophytic forest. Where a
forest is cut over for certain species alone, the undisturbed trees soon take
full possession, though the causes effective in the beginning will ultimately
restore the original facies in many instances. Such successions are
anomalous, and will be treated under that head.
309. Succession by cultivation. The clearing of forests and the
“breaking” of grassland for cultivation destroy the original vegetation; the
succession.
308. Succession in lumbered areas. Commercial lumbering, especially
where practiced for wood-pulp as well as for timber, results in complete or
nearly complete destruction of the vegetation by removal and the change
from diffuse light to sunlight, or by the action of erosion upon the exposed
surface. In the first place, short mesophytic successions will result; in the
second, the successions will be long and complex, passing through
decreasingly xerophytic conditions to a stable mesophytic forest. Where a
forest is cut over for certain species alone, the undisturbed trees soon take
full possession, though the causes effective in the beginning will ultimately
restore the original facies in many instances. Such successions are
anomalous, and will be treated under that head.
309. Succession by cultivation. The clearing of forests and the
“breaking” of grassland for cultivation destroy the original vegetation; the
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temporary or permanent abandonment of cultivated fields then permits the
entrance of ruderal species, which are the pioneers of new successions. This
phenomenon takes place annually in fields after harvest, resulting in the
secondary formations of Warming, in which practically the same species
reappear year after year. In fields that lie fallow for several years, or are
permanently abandoned, the first ruderal plants are displaced by
newcomers, or certain of them become dominant at the expense of others.
In a few years, these are crowded out by invaders from the adjacent
formations, and the field is ultimately reclaimed by the original vegetation,
unless this has entirely disappeared from the region. The number of stages
depends chiefly upon whether the final formation is to be grassland or
woodland. Other activities of man, such as the construction of buildings,
roads, railways, canals, etc., remove the native vegetation, and make room
for the rapid development of ruderal formations. In and about cities, where
the original formations have entirely disappeared, the chance for succession
is remote, and the initial ruderal stages become more or less stabilized.
Elsewhere the usual successions are established, and the ruderal formation
finally gives way to the dominant type. In mountain and desert regions,
where ruderal plants are rare or lacking, their place is taken by subruderal
forms, species of the native vegetation capable of rapid movement in them.
These, like ruderal plants, are gradually replaced by other native species of
less mobility, but of greater persistence, resulting in a short succession
operating often within a single formation. From the nature of cultivated
plants, succession after cultivation generally operates within the mesophytic
series.
310. Succession by drainage. Successions of this kind show much the
same stages as are found in those due to flooding. They proceed from
aquatic or swamp formations to mesophytic termini, either grassland or
woodland. When drainage takes place rapidly and completely, the pioneer
stages are usually xerophytic; cases of this sort, however, are infrequent.
311. Succession by irrigation. Irrigation produces short successions of
peculiar stamp along the courses of irrigating canals and ditches, and in the
vicinity of reservoirs. These are recent, as a rule, and are usually found in
the midst of cultivated lands, so that their complete history is still a matter
of conjecture. The original xerophytes are forced out not only by the
disturbance of the soil, but also by its increased water-content. A few of
them often thrive under the new conditions, and, together with the usual
entrance of ruderal species, which are the pioneers of new successions. This
phenomenon takes place annually in fields after harvest, resulting in the
secondary formations of Warming, in which practically the same species
reappear year after year. In fields that lie fallow for several years, or are
permanently abandoned, the first ruderal plants are displaced by
newcomers, or certain of them become dominant at the expense of others.
In a few years, these are crowded out by invaders from the adjacent
formations, and the field is ultimately reclaimed by the original vegetation,
unless this has entirely disappeared from the region. The number of stages
depends chiefly upon whether the final formation is to be grassland or
woodland. Other activities of man, such as the construction of buildings,
roads, railways, canals, etc., remove the native vegetation, and make room
for the rapid development of ruderal formations. In and about cities, where
the original formations have entirely disappeared, the chance for succession
is remote, and the initial ruderal stages become more or less stabilized.
Elsewhere the usual successions are established, and the ruderal formation
finally gives way to the dominant type. In mountain and desert regions,
where ruderal plants are rare or lacking, their place is taken by subruderal
forms, species of the native vegetation capable of rapid movement in them.
These, like ruderal plants, are gradually replaced by other native species of
less mobility, but of greater persistence, resulting in a short succession
operating often within a single formation. From the nature of cultivated
plants, succession after cultivation generally operates within the mesophytic
series.
310. Succession by drainage. Successions of this kind show much the
same stages as are found in those due to flooding. They proceed from
aquatic or swamp formations to mesophytic termini, either grassland or
woodland. When drainage takes place rapidly and completely, the pioneer
stages are usually xerophytic; cases of this sort, however, are infrequent.
311. Succession by irrigation. Irrigation produces short successions of
peculiar stamp along the courses of irrigating canals and ditches, and in the
vicinity of reservoirs. These are recent, as a rule, and are usually found in
the midst of cultivated lands, so that their complete history is still a matter
of conjecture. The original xerophytes are forced out not only by the
disturbance of the soil, but also by its increased water-content. A few of
them often thrive under the new conditions, and, together with the usual
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ruderal plants and a large number of lowland mesophytes and amphibious
forms derived from the banks of the parent stream, constitute a
heterogeneous association. This is doubtless to be regarded as an initial
stage of a succession, but it is an open question whether the succession will
early be stabilized as a new formation, or whether the original vegetation
will sooner or later be reestablished under somewhat mesophytic
conditions. From the number of mesophytes and from the behavior of
valleys, it seems certain that the banks of such canals will ultimately be
occupied by a formation more mesophytic than hydrophytic, into which
some of the surrounding xerophytes of plastic nature have been adopted.
312. Anomalous successions are those in which the physical change in
the habitat is relatively slight, resulting in a displacement of the ultimate
stage, or the disturbance of the usual sequence, merely, instead of the
destruction and reconstruction of a formation, or the gradual development
of a new series of stages on new soil. In nature, the ultimate grass or forest
stage of a normal succession is often replaced by a similar formation,
especially if the facies be few or single. It is evident that certain trees
naturally replace others in the last stages of a forest succession, without
making the latter anomalous. The last occurs only when a normal stage is
replaced by one belonging properly to an entirely different succession, as
when a coniferous forest replaces a deciduous one in a hardwood region.
The presence and development of such successions can be determined only
after the normal types are known. The interpolation of a foreign stage in a
natural succession, or a change of direction, by which a succession that is
mesotropic again becomes hydrophytic, is easily explained when it is the
result of artificial agents, as is often the case. In nature, anomalous
successions are commonly the result of a slow backward and forward swing
of climatic conditions.
313. Perfect and imperfect successions. A normal succession will
regularly be perfect; it passes in the usual sequence from initial to ultimate
conditions without interruption or omission. Imperfect succession results
when one or more of the ordinary stages is omitted anywhere in the course,
and a later stage appears before its turn. It will occur at any time when a
new or denuded habitat becomes so surrounded by other vegetation that the
formations which usually furnish the next invaders are unable to do so, or
when the abundance and mobility of certain species enable them to take
possession before their proper turn, and to the exclusion of the regular
forms derived from the banks of the parent stream, constitute a
heterogeneous association. This is doubtless to be regarded as an initial
stage of a succession, but it is an open question whether the succession will
early be stabilized as a new formation, or whether the original vegetation
will sooner or later be reestablished under somewhat mesophytic
conditions. From the number of mesophytes and from the behavior of
valleys, it seems certain that the banks of such canals will ultimately be
occupied by a formation more mesophytic than hydrophytic, into which
some of the surrounding xerophytes of plastic nature have been adopted.
312. Anomalous successions are those in which the physical change in
the habitat is relatively slight, resulting in a displacement of the ultimate
stage, or the disturbance of the usual sequence, merely, instead of the
destruction and reconstruction of a formation, or the gradual development
of a new series of stages on new soil. In nature, the ultimate grass or forest
stage of a normal succession is often replaced by a similar formation,
especially if the facies be few or single. It is evident that certain trees
naturally replace others in the last stages of a forest succession, without
making the latter anomalous. The last occurs only when a normal stage is
replaced by one belonging properly to an entirely different succession, as
when a coniferous forest replaces a deciduous one in a hardwood region.
The presence and development of such successions can be determined only
after the normal types are known. The interpolation of a foreign stage in a
natural succession, or a change of direction, by which a succession that is
mesotropic again becomes hydrophytic, is easily explained when it is the
result of artificial agents, as is often the case. In nature, anomalous
successions are commonly the result of a slow backward and forward swing
of climatic conditions.
313. Perfect and imperfect successions. A normal succession will
regularly be perfect; it passes in the usual sequence from initial to ultimate
conditions without interruption or omission. Imperfect succession results
when one or more of the ordinary stages is omitted anywhere in the course,
and a later stage appears before its turn. It will occur at any time when a
new or denuded habitat becomes so surrounded by other vegetation that the
formations which usually furnish the next invaders are unable to do so, or
when the abundance and mobility of certain species enable them to take
possession before their proper turn, and to the exclusion of the regular
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stage. Incomplete successions are of great significance, inasmuch as they
indicate that the stages of a succession are often due more to biological than
to physical causes, the proximity and mobility of the adjacent species being
more determinative than the physical factors. Subalpine gravel slides
regularly pass through the rosette, mat, turf, thicket, woodland, and forest
stages; occasionally, however, they pass immediately from the rosette, or
mat condition, to an aspen thicket which represents the next to the last
stage. Such successions are by no means infrequent in hilly and montane
regions; in regions physiographically more mature or stable, perfect
successions are almost invariably the rule.
Fig. 65. Half gravel slide formation (Elymus-Muhlenbergia-chalicium), stage IV of the talus
succession.
indicate that the stages of a succession are often due more to biological than
to physical causes, the proximity and mobility of the adjacent species being
more determinative than the physical factors. Subalpine gravel slides
regularly pass through the rosette, mat, turf, thicket, woodland, and forest
stages; occasionally, however, they pass immediately from the rosette, or
mat condition, to an aspen thicket which represents the next to the last
stage. Such successions are by no means infrequent in hilly and montane
regions; in regions physiographically more mature or stable, perfect
successions are almost invariably the rule.
Fig. 65. Half gravel slide formation (Elymus-Muhlenbergia-chalicium), stage IV of the talus
succession.
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314. Stabilization. It may be stated as a general principle that vegetation
moves constantly and gradually toward stabilization. Each successive stage
modifies the physical factors, and dominates the habitat more and more, in
such a way that the latter seems to respond to the formation rather than this
to the habitat. The more advanced the succession, i. e., the degree of
stabilization, the greater the climatic or physiographic change necessary to
disturb it, with the result that such disturbances are much more frequent in
the earlier stages than in the later development. Constant, gradual
movement toward a stable formation is characteristic of continuous
succession. Contrasted with this is intermittent succession, in which the
succession swings for a time in one direction, from xerophytic to
mesophytic for example, and then moves in the opposite direction, often
passing through the same stages. This phenomenon usually is characteristic
only of the less stable stages, and is generally produced by a climatic swing,
in which a series of hot or dry years is followed by one of cold or wet years,
or the reverse. The same effect upon a vast scale is produced by alternate
elevation and subsidence, but these operate through such great periods of
time that one can not trace, but can only conjecture their effects. A normal
continuous succession frequently changes its direction of movement, or its
type, in transition regions or in areas where the outposts of a new flora are
rapidly advancing, as in wide mesophytic valleys that run down into or
traverse plains. Here the change is often sudden, and grass and desert
formations are replaced by thickets and forests, resulting in abrupt
succession. Species guilds are typical examples of this. More rarely, a stage
foreign to the succession will be interpolated, replacing a normal stage, or
slipping in between two such, though finally disappearing before the next
regular formation. This may be distinguished as interpolated succession.
The apparent terminus of all stabilization is the forest, on account of the
thoroughness with which it controls the habitat. A close examination of
vegetation, however, will show that its stable terms are dependent in the
first degree upon the character of the region in which the formation is
indigenous. It is obviously impossible that successions in desert lands, in
polar barrens, or upon alpine stretches should terminate in forest stages. In
these, grassland must be the ultimate condition, except in those extreme
habitats, alpine and polar, where mosses and lichens represent the highest
type of existing vegetation. Forests are ultimate for all successions in
habitats belonging to a region generally wooded, while grassland represents
moves constantly and gradually toward stabilization. Each successive stage
modifies the physical factors, and dominates the habitat more and more, in
such a way that the latter seems to respond to the formation rather than this
to the habitat. The more advanced the succession, i. e., the degree of
stabilization, the greater the climatic or physiographic change necessary to
disturb it, with the result that such disturbances are much more frequent in
the earlier stages than in the later development. Constant, gradual
movement toward a stable formation is characteristic of continuous
succession. Contrasted with this is intermittent succession, in which the
succession swings for a time in one direction, from xerophytic to
mesophytic for example, and then moves in the opposite direction, often
passing through the same stages. This phenomenon usually is characteristic
only of the less stable stages, and is generally produced by a climatic swing,
in which a series of hot or dry years is followed by one of cold or wet years,
or the reverse. The same effect upon a vast scale is produced by alternate
elevation and subsidence, but these operate through such great periods of
time that one can not trace, but can only conjecture their effects. A normal
continuous succession frequently changes its direction of movement, or its
type, in transition regions or in areas where the outposts of a new flora are
rapidly advancing, as in wide mesophytic valleys that run down into or
traverse plains. Here the change is often sudden, and grass and desert
formations are replaced by thickets and forests, resulting in abrupt
succession. Species guilds are typical examples of this. More rarely, a stage
foreign to the succession will be interpolated, replacing a normal stage, or
slipping in between two such, though finally disappearing before the next
regular formation. This may be distinguished as interpolated succession.
The apparent terminus of all stabilization is the forest, on account of the
thoroughness with which it controls the habitat. A close examination of
vegetation, however, will show that its stable terms are dependent in the
first degree upon the character of the region in which the formation is
indigenous. It is obviously impossible that successions in desert lands, in
polar barrens, or upon alpine stretches should terminate in forest stages. In
these, grassland must be the ultimate condition, except in those extreme
habitats, alpine and polar, where mosses and lichens represent the highest
type of existing vegetation. Forests are ultimate for all successions in
habitats belonging to a region generally wooded, while grassland represents
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the terminus of prairie and plains successions as well as of many arctic-
alpine ones.
CAUSES AND REACTIONS
315. The initial cause of a succession must be sought in a physical
change in the habitat; its continuance depends upon the reaction which each
stage of vegetation exerts upon the physical factors which constitute the
habitat. A single exception to this is found in anomalous successions, where
the change of formation often hinges upon the appearance of remote or
foreign disseminules. The causes which initiate successions have already
been considered; they may be summarized as follows: (1) weathering, (2)
erosion, (3) elevation, (4) subsidence, (5) climatic changes, (6) artificial
changes. The effect of succeeding stages of vegetation upon a new or
denuded habitat usually finds expression in a change of the habitat with
respect to a particular factor, and in a definite direction. Often, there is a
primary reaction, and one or more secondary ones, which are corollaries of
it. Rarely, there are two or more coordinate reactions. The general ways in
which vegetation reacts upon the habitat are the following: (1) by
preventing weathering, (2) by binding aeolian soils, (3) by reducing run-off
and preventing erosion, (4) by filling with silt and plant remains, (5) by
enriching the soil, (6) by exhausting the soil, (7) by accumulating humus,
(8) by modifying atmospheric factors. The direction of the movement of a
succession is the immediate result of its reaction. From the fundamental
nature of vegetation, it must be expressed in terms of water-content. The
reaction is often so great that the habitat undergoes a profound change in
the course of the succession, changing from hydrophytic to mesophytic or
xerophytic, or the reverse. This is characteristic of newly formed or exposed
soils. Such successions are xerotropic, mesotropic, or hydrotropic,
according to the ultimate condition of the habitat. When the reaction is less
marked, the type of habitat does not change materially, and the successions
are xerostatic, mesostatic, or hydrostatic, depending upon the water-content.
Such conditions obtain for the most part only in denuded habitats.
316. Succession by preventing weathering. Reactions of this nature
occur especially in alpine and boreal regions, in the earlier stages of lichen-
moss successions. They are typical of igneous and metamorphic rocks in
alpine ones.
CAUSES AND REACTIONS
315. The initial cause of a succession must be sought in a physical
change in the habitat; its continuance depends upon the reaction which each
stage of vegetation exerts upon the physical factors which constitute the
habitat. A single exception to this is found in anomalous successions, where
the change of formation often hinges upon the appearance of remote or
foreign disseminules. The causes which initiate successions have already
been considered; they may be summarized as follows: (1) weathering, (2)
erosion, (3) elevation, (4) subsidence, (5) climatic changes, (6) artificial
changes. The effect of succeeding stages of vegetation upon a new or
denuded habitat usually finds expression in a change of the habitat with
respect to a particular factor, and in a definite direction. Often, there is a
primary reaction, and one or more secondary ones, which are corollaries of
it. Rarely, there are two or more coordinate reactions. The general ways in
which vegetation reacts upon the habitat are the following: (1) by
preventing weathering, (2) by binding aeolian soils, (3) by reducing run-off
and preventing erosion, (4) by filling with silt and plant remains, (5) by
enriching the soil, (6) by exhausting the soil, (7) by accumulating humus,
(8) by modifying atmospheric factors. The direction of the movement of a
succession is the immediate result of its reaction. From the fundamental
nature of vegetation, it must be expressed in terms of water-content. The
reaction is often so great that the habitat undergoes a profound change in
the course of the succession, changing from hydrophytic to mesophytic or
xerophytic, or the reverse. This is characteristic of newly formed or exposed
soils. Such successions are xerotropic, mesotropic, or hydrotropic,
according to the ultimate condition of the habitat. When the reaction is less
marked, the type of habitat does not change materially, and the successions
are xerostatic, mesostatic, or hydrostatic, depending upon the water-content.
Such conditions obtain for the most part only in denuded habitats.
316. Succession by preventing weathering. Reactions of this nature
occur especially in alpine and boreal regions, in the earlier stages of lichen-
moss successions. They are typical of igneous and metamorphic rocks in
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which disintegration regularly precedes decomposition. The influence of the
vegetation is best seen in the lichen stages, where the crustose forms make a
compact layer, which diminishes the effect of the atmospheric factors
producing disintegration. In alpine regions especially, this protection is so
perfect that the crustose lichens may almost be regarded as the last stage of
a succession. There are no recorded observations which bear upon this
point, but it seems certain that the pioneer rock lichens, Lecanora, Lecidea,
Biatora, Buellia, and Acarospora, cover alpine rocks for decades, if not for
centuries. Ultimately, however, the slow decomposition of the rock surface
beneath the thallus has its effect. Tiny furrows and pockets are formed, in
which water accumulates to carry on its ceaseless work, and the compact
crustose covering is finally ruptured, permitting the entrance of foliose
forms. The latter, like the mosses, doubtless protect rock surfaces,
especially those of the softer rocks, in a slight degree against the influence
of weathering, but this is more than offset by their activity in hastening
decomposition, and thus preparing a field for invasion. Rocks and boulders
(petria, petrodia, phellia) furnish the best examples of this reaction; cliffs
(cremnia) usually have a lichen covering on their faces, while the forces
which produce disintegration operate from above or below.
vegetation is best seen in the lichen stages, where the crustose forms make a
compact layer, which diminishes the effect of the atmospheric factors
producing disintegration. In alpine regions especially, this protection is so
perfect that the crustose lichens may almost be regarded as the last stage of
a succession. There are no recorded observations which bear upon this
point, but it seems certain that the pioneer rock lichens, Lecanora, Lecidea,
Biatora, Buellia, and Acarospora, cover alpine rocks for decades, if not for
centuries. Ultimately, however, the slow decomposition of the rock surface
beneath the thallus has its effect. Tiny furrows and pockets are formed, in
which water accumulates to carry on its ceaseless work, and the compact
crustose covering is finally ruptured, permitting the entrance of foliose
forms. The latter, like the mosses, doubtless protect rock surfaces,
especially those of the softer rocks, in a slight degree against the influence
of weathering, but this is more than offset by their activity in hastening
decomposition, and thus preparing a field for invasion. Rocks and boulders
(petria, petrodia, phellia) furnish the best examples of this reaction; cliffs
(cremnia) usually have a lichen covering on their faces, while the forces
which produce disintegration operate from above or below.
Page 333
Fig. 66. Thicket formation (Quercus-Holodiscus-driodium), stage V of the talus succession.
317. Succession by binding aeolian soils. Dunes (thinia) are classic
examples of the reaction of pioneer vegetation upon habitats of wind-borne
sand. The initial formations in such places consist exclusively of sand-
binders, plants with masses of fibrous roots, and usually also with strong
rootstalks, long, erect leaves, and a vigorous apical growth. They are almost
exclusively perennial grasses and sedges, possessing the unique property of
pushing up rapidly through a covering of sand. They react by fixing the
sand with their roots, thus preventing its blowing about, and also by
catching the shifting particles among their culms and leaves, forming a tiny
area of stabilization, in which the next generation can establish a foothold.
The gradual accumulation of vegetable detritus serves also to enrich the
soil, and makes possible the advent of species requiring better nourishment.
Blowouts (anemia) are almost exact duplicates of dunes in so far as the
steps of revegetation are concerned; while one is a hollow, and the other a
317. Succession by binding aeolian soils. Dunes (thinia) are classic
examples of the reaction of pioneer vegetation upon habitats of wind-borne
sand. The initial formations in such places consist exclusively of sand-
binders, plants with masses of fibrous roots, and usually also with strong
rootstalks, long, erect leaves, and a vigorous apical growth. They are almost
exclusively perennial grasses and sedges, possessing the unique property of
pushing up rapidly through a covering of sand. They react by fixing the
sand with their roots, thus preventing its blowing about, and also by
catching the shifting particles among their culms and leaves, forming a tiny
area of stabilization, in which the next generation can establish a foothold.
The gradual accumulation of vegetable detritus serves also to enrich the
soil, and makes possible the advent of species requiring better nourishment.
Blowouts (anemia) are almost exact duplicates of dunes in so far as the
steps of revegetation are concerned; while one is a hollow, and the other a
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hill, in both the reaction operates upon a wind-swept slope. Sand-hills
(amathia) and deserts (eremia) show similar though less marked reactions,
except where they exhibit typical inland dunes. Sand-binders, while usually
classed as xerophytic or halophytic, are in reality dissophytes. Their roots
grow more or less superficially in moist sand, and are morphologically
mesophytic while their leaves bear the stamp of xerophytes. The direction
of movement in successions of this kind is normally from xerophytes to
mesophytes, i. e., it is mesotropic. In sand-hills and deserts, the succession
operates wholly within the xerophytic (dissophytic) series. Along seacoasts,
the mesophytic terminus is regularly forest, except where forests are
remote, when it is grassland.
318. Succession by reducing run-off and erosion. All bare or denuded
habitats that have an appreciable slope are subject to erosion by surface
water. The rapidity and degree of erosion depend upon the amount of
rainfall, the inclination of the slope, and the structure of the surface soil.
Regions of excessive rainfall, even where the slope is slight, show great,
though somewhat uniform erosion; hill and mountain are deeply eroded
even when the rainfall is small. Slopes consisting of compact eugeogenous
soils, notwithstanding the marked adhesion of the particles, are much
eroded where the rainfall is great, on account of the excessive run-off.
Porous dysgeogenous soils, on the contrary, absorb most of the rainfall; the
run-off is small and erosion slight, except where the slope is great, a rare
condition on account of the imperfect cohesion of the particles. In compact
soils, the plants of the initial formations not merely break the impact of the
raindrops, but, what is much more important, they delay the downward
movement of the water, and produce numberless tiny streams. The delayed
water is largely absorbed by the soil, and the reduction of the run-off
prevents the formation of rills of sufficient size to cause erosion. As in
dunes, such plants are usually perennial grasses, though composites are
frequent; the root system is, however, more deeply seated, and a main or tap
root is often present. On sand and gravel slopes, the loose texture of the soil
results generally in the production of sand-binders with fibrous roots.
Unlike dunes, such slopes exhibit a large number of mats and rosettes with
tap-roots, which are effective in preventing the slipping or washing of the
sand, and run little danger of being covered, as is the case with
duneformers. In both instances, each pioneer plant serves as a center of
comparative stabilization for the establishment of its own offspring, and of
(amathia) and deserts (eremia) show similar though less marked reactions,
except where they exhibit typical inland dunes. Sand-binders, while usually
classed as xerophytic or halophytic, are in reality dissophytes. Their roots
grow more or less superficially in moist sand, and are morphologically
mesophytic while their leaves bear the stamp of xerophytes. The direction
of movement in successions of this kind is normally from xerophytes to
mesophytes, i. e., it is mesotropic. In sand-hills and deserts, the succession
operates wholly within the xerophytic (dissophytic) series. Along seacoasts,
the mesophytic terminus is regularly forest, except where forests are
remote, when it is grassland.
318. Succession by reducing run-off and erosion. All bare or denuded
habitats that have an appreciable slope are subject to erosion by surface
water. The rapidity and degree of erosion depend upon the amount of
rainfall, the inclination of the slope, and the structure of the surface soil.
Regions of excessive rainfall, even where the slope is slight, show great,
though somewhat uniform erosion; hill and mountain are deeply eroded
even when the rainfall is small. Slopes consisting of compact eugeogenous
soils, notwithstanding the marked adhesion of the particles, are much
eroded where the rainfall is great, on account of the excessive run-off.
Porous dysgeogenous soils, on the contrary, absorb most of the rainfall; the
run-off is small and erosion slight, except where the slope is great, a rare
condition on account of the imperfect cohesion of the particles. In compact
soils, the plants of the initial formations not merely break the impact of the
raindrops, but, what is much more important, they delay the downward
movement of the water, and produce numberless tiny streams. The delayed
water is largely absorbed by the soil, and the reduction of the run-off
prevents the formation of rills of sufficient size to cause erosion. As in
dunes, such plants are usually perennial grasses, though composites are
frequent; the root system is, however, more deeply seated, and a main or tap
root is often present. On sand and gravel slopes, the loose texture of the soil
results generally in the production of sand-binders with fibrous roots.
Unlike dunes, such slopes exhibit a large number of mats and rosettes with
tap-roots, which are effective in preventing the slipping or washing of the
sand, and run little danger of being covered, as is the case with
duneformers. In both instances, each pioneer plant serves as a center of
comparative stabilization for the establishment of its own offspring, and of
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such invaders as find their way in. From the nature of these, slopes almost
invariably pass through grassland stages before finding their termini in
thickets or forests. Bad lands (tiria) furnish the most striking examples of
eroded habitats. The rainfall in the bad lands of Nebraska and South Dakota
is small (300 mm.); yet the steepness of the slope and the compactness of
the soil render erosion so extreme that it is all but impossible for plants to
obtain a foothold. Their reaction is practically negligible, and the vegetation
passes the pioneer stages only in the relatively stable valleys. Mountain
slopes (ancia), and ridges and hills (lophia) are readily eroded in new or
denuded areas. This is especially true of hill and mountain regions which
have been stripped of their forest or thicket cover by fires, lumbering,
cultivation, or grazing. Where the erosion is slight, the resulting succession
may show initial xerophytic stages, or it may be completely mesostatic.
Excessively eroded habitats are xerostatic, as in the case of bad lands, or,
more frequently, they are mesotropic, passing first through a long series of
xerophytic formations. Sandbars (cheradia, syrtidia) should be considered
here, though they are eroded by currents and waves, and not by run-off.
They are fixed and built up by sand-binding grasses and sedges, usually of a
hydrophytic nature, and pass ultimately into mesophytic forest.
319. Succession by filling with silt and plant remains. All aquatic
habitats into which silt, wash, or other detritus is borne by streams, currents,
floods, waves, or tides are slowly shallowed by the action of the water
plants present. These not only check the movement of the water, thus
greatly decreasing its carrying power, and causing the deposition of a part
or all of its load, but they also retain and fix the particles deposited. In
accordance with the rule, each plant becomes the center of a stabilizing
area, which rises faster than the rest of the floor, producing the well-known
hummocks of lagoons and swamps. All aquatics produce this reaction. It is
more pronounced in submerged and amphibious forms than in floating
ones, and it takes place more rapidly with greatly branched or dissected
plants than with others. In pools (tiphia) and lakes (limnia), debouching
streams and surface waters deposit their loads in consequence of the check
exerted by the still water and the marginal vegetation, and delta-like
marshes are quickly built up by filling. Springs (crenia) likewise form
marshes where they gush forth in sands, the removal of which is impeded
by vegetation. The flood plains and deltas of rivers show a similar reaction.
The heavily laden flood waters are checked by the vegetation of meadows
invariably pass through grassland stages before finding their termini in
thickets or forests. Bad lands (tiria) furnish the most striking examples of
eroded habitats. The rainfall in the bad lands of Nebraska and South Dakota
is small (300 mm.); yet the steepness of the slope and the compactness of
the soil render erosion so extreme that it is all but impossible for plants to
obtain a foothold. Their reaction is practically negligible, and the vegetation
passes the pioneer stages only in the relatively stable valleys. Mountain
slopes (ancia), and ridges and hills (lophia) are readily eroded in new or
denuded areas. This is especially true of hill and mountain regions which
have been stripped of their forest or thicket cover by fires, lumbering,
cultivation, or grazing. Where the erosion is slight, the resulting succession
may show initial xerophytic stages, or it may be completely mesostatic.
Excessively eroded habitats are xerostatic, as in the case of bad lands, or,
more frequently, they are mesotropic, passing first through a long series of
xerophytic formations. Sandbars (cheradia, syrtidia) should be considered
here, though they are eroded by currents and waves, and not by run-off.
They are fixed and built up by sand-binding grasses and sedges, usually of a
hydrophytic nature, and pass ultimately into mesophytic forest.
319. Succession by filling with silt and plant remains. All aquatic
habitats into which silt, wash, or other detritus is borne by streams, currents,
floods, waves, or tides are slowly shallowed by the action of the water
plants present. These not only check the movement of the water, thus
greatly decreasing its carrying power, and causing the deposition of a part
or all of its load, but they also retain and fix the particles deposited. In
accordance with the rule, each plant becomes the center of a stabilizing
area, which rises faster than the rest of the floor, producing the well-known
hummocks of lagoons and swamps. All aquatics produce this reaction. It is
more pronounced in submerged and amphibious forms than in floating
ones, and it takes place more rapidly with greatly branched or dissected
plants than with others. In pools (tiphia) and lakes (limnia), debouching
streams and surface waters deposit their loads in consequence of the check
exerted by the still water and the marginal vegetation, and delta-like
marshes are quickly built up by filling. Springs (crenia) likewise form
marshes where they gush forth in sands, the removal of which is impeded
by vegetation. The flood plains and deltas of rivers show a similar reaction.
The heavily laden flood waters are checked by the vegetation of meadows
Page 336
and marshes, and deposit most of their load. The banks of streams (ochthia)
and of ditches (taphria) are often built up in the same fashion by the action
of the marginal vegetation upon the current. The presence of marginal
vegetation often determines the checking or deflecting of the current in such
a way as to initiate meanders, while natural levees owe their origin to it, in
part at least. Along low seacoasts, waves and tides hasten the deposit of
river-borne detritus, causing the water to spread over the lowlands and form
swamps. They often throw back also the sediment that has been deposited
in the sea, the marsh vegetation acting as a filter in both cases. Successions
of the kind indicated above are regularly mesotropic. Where the soil is
sandy, and the filling-up process sufficiently great, or where salts or humus
occur in excess, xerophytic formations result. In certain cases, these
successions appear to be permanently hydrostatic, changing merely from
floating or submerged to amphibious conditions, but this is probably due to
the slowness of the reaction. As a rule, the accumulation of plant remains is
relatively slight, and plays an unimportant part in the reaction. In peat bogs
and other extensive swamps, the amount of organic matter is excessive, and
plays an important role in the building up of the swamp bed.
and of ditches (taphria) are often built up in the same fashion by the action
of the marginal vegetation upon the current. The presence of marginal
vegetation often determines the checking or deflecting of the current in such
a way as to initiate meanders, while natural levees owe their origin to it, in
part at least. Along low seacoasts, waves and tides hasten the deposit of
river-borne detritus, causing the water to spread over the lowlands and form
swamps. They often throw back also the sediment that has been deposited
in the sea, the marsh vegetation acting as a filter in both cases. Successions
of the kind indicated above are regularly mesotropic. Where the soil is
sandy, and the filling-up process sufficiently great, or where salts or humus
occur in excess, xerophytic formations result. In certain cases, these
successions appear to be permanently hydrostatic, changing merely from
floating or submerged to amphibious conditions, but this is probably due to
the slowness of the reaction. As a rule, the accumulation of plant remains is
relatively slight, and plays an unimportant part in the reaction. In peat bogs
and other extensive swamps, the amount of organic matter is excessive, and
plays an important role in the building up of the swamp bed.
Page 337
Fig. 67. Pine forest formation (Pinus-xerohylium), stage VI of the talus succession.
320. Succession by enriching the soil. This reaction occurs to some
degree in the great majority of all successions. The relatively insignificant
lichens and mosses produce this result upon the most barren rocks, while
the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it
in marked progression. The reaction consists chiefly in the incorporation of
the decomposed remains of each generation and each stage in the soil. A
very important part is played by the mechanical and chemical action of the
roots in breaking up the soil particles, and in changing them into soluble
substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play
a large part in increasing the nutrition-content of the soil, but the extent to
which they are effective in succession is completely unknown. The changes
in the color, texture, and food value of the soil in passing from the initial to
ultimate stages of a normal succession are well known, and have led many
to think them the efficient reactions of such successions. It seems almost
320. Succession by enriching the soil. This reaction occurs to some
degree in the great majority of all successions. The relatively insignificant
lichens and mosses produce this result upon the most barren rocks, while
the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it
in marked progression. The reaction consists chiefly in the incorporation of
the decomposed remains of each generation and each stage in the soil. A
very important part is played by the mechanical and chemical action of the
roots in breaking up the soil particles, and in changing them into soluble
substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play
a large part in increasing the nutrition-content of the soil, but the extent to
which they are effective in succession is completely unknown. The changes
in the color, texture, and food value of the soil in passing from the initial to
ultimate stages of a normal succession are well known, and have led many
to think them the efficient reactions of such successions. It seems almost
Page 338
certain, however, that this is merely a concomitant, and that, even in
anomalous successions where facies replace each other without obvious
reasons, the reactions are concerned more with water-content, light, and
humidity than with the food-content of the soil.
321. Succession by exhausting the soil. This is a reaction not at all
understood as yet in nature. A number of phenomena, such as the “fairy
rings” of mushrooms and other fungi, the peripheral growth and central
decay of lichens, Lecanora, Placodium, Parmelia, and of matforming
grasses, such as Muhlenbergia, and the circular advance of the rootstalk
plants, indicate that certain plants at least withdraw much of the available
supply of some essential soil element, and are forced to move away from
the exhausted area. It is probable that the constant shifting of the individuals
of a formation year after year, a phenomenon to be discussed under
alternation, has some connection with this. It will be impossible to establish
such a relation, however, until the facts are exactly determined by the
method of quadrat statistics. So far as native formations are concerned,
there can not be the slightest question that prairies and forests have existed
over the same area for centuries without impoverishing the soil in the least
degree, a conclusion which is even more certain for the open vegetation of
deserts and plains. With culture formations, the case is quite different. The
exhaustion of the soil by continuous or intensive cultivation is a matter of
common experience in all lands settled for a long period. Calcium,
phosphorus, and nitrogen compounds especially are used up by crops, and
must be supplied artificially. The reason for this difference in reaction
between native and culture formations seems evident. In harvesting, not
merely the grain, but the stems and leaves, and in gardening often the root
also, are removed, so that the plant makes little or no return to the soil. In
nature, annual plants return to the ground every year all the solid matter of
roots, stems, leaves, and fruits, with the exception of the relatively small
number of seeds that germinate. Perennial herbs return everything but the
persistent underground parts. Shrubs and trees replace annually an immense
amount of material used in leaves and fruits, and sooner or later, by the
gradual decay of the individuals or by the destruction of the whole
formation, they restore all that they have taken from the soil. This balance is
further maintained to an important degree by the activity of the roots, which
take from the deep-seated layers of the soil the crude materials necessary
for the formation of leaves and fruits. Upon the fall and decay of these, their
anomalous successions where facies replace each other without obvious
reasons, the reactions are concerned more with water-content, light, and
humidity than with the food-content of the soil.
321. Succession by exhausting the soil. This is a reaction not at all
understood as yet in nature. A number of phenomena, such as the “fairy
rings” of mushrooms and other fungi, the peripheral growth and central
decay of lichens, Lecanora, Placodium, Parmelia, and of matforming
grasses, such as Muhlenbergia, and the circular advance of the rootstalk
plants, indicate that certain plants at least withdraw much of the available
supply of some essential soil element, and are forced to move away from
the exhausted area. It is probable that the constant shifting of the individuals
of a formation year after year, a phenomenon to be discussed under
alternation, has some connection with this. It will be impossible to establish
such a relation, however, until the facts are exactly determined by the
method of quadrat statistics. So far as native formations are concerned,
there can not be the slightest question that prairies and forests have existed
over the same area for centuries without impoverishing the soil in the least
degree, a conclusion which is even more certain for the open vegetation of
deserts and plains. With culture formations, the case is quite different. The
exhaustion of the soil by continuous or intensive cultivation is a matter of
common experience in all lands settled for a long period. Calcium,
phosphorus, and nitrogen compounds especially are used up by crops, and
must be supplied artificially. The reason for this difference in reaction
between native and culture formations seems evident. In harvesting, not
merely the grain, but the stems and leaves, and in gardening often the root
also, are removed, so that the plant makes little or no return to the soil. In
nature, annual plants return to the ground every year all the solid matter of
roots, stems, leaves, and fruits, with the exception of the relatively small
number of seeds that germinate. Perennial herbs return everything but the
persistent underground parts. Shrubs and trees replace annually an immense
amount of material used in leaves and fruits, and sooner or later, by the
gradual decay of the individuals or by the destruction of the whole
formation, they restore all that they have taken from the soil. This balance is
further maintained to an important degree by the activity of the roots, which
take from the deep-seated layers of the soil the crude materials necessary
for the formation of leaves and fruits. Upon the fall and decay of these, their
Page 339
materials are incorporated with the upper layers of the formation floor, from
which they may be absorbed by the undergrowth, or find their way again
into the layers permeated by the tree roots. From the universal occurrence
of weeds in cultivated regions, the pioneers in impoverished or exhausted
fields are uniformly ruderal plants. As is well known, the seed production
and ecesis of these forms are such that they take possession quickly and
completely, while their demands upon the soil are of such a nature that the
most sterile field can rapidly be covered by a vigorous growth of weeds. As
indicated elsewhere, ruderal formations ultimately yield to the native
vegetation, though in regions so completely given over to culture that native
formations are lacking or remote, it is probable that successions reach their
final stage within the group of ruderal plants.
Fig. 68. Spruce forest formation (Picea-Pseudotsuga-hylium), stage VII, the ultimate stage of the
talus succession.
which they may be absorbed by the undergrowth, or find their way again
into the layers permeated by the tree roots. From the universal occurrence
of weeds in cultivated regions, the pioneers in impoverished or exhausted
fields are uniformly ruderal plants. As is well known, the seed production
and ecesis of these forms are such that they take possession quickly and
completely, while their demands upon the soil are of such a nature that the
most sterile field can rapidly be covered by a vigorous growth of weeds. As
indicated elsewhere, ruderal formations ultimately yield to the native
vegetation, though in regions so completely given over to culture that native
formations are lacking or remote, it is probable that successions reach their
final stage within the group of ruderal plants.
Fig. 68. Spruce forest formation (Picea-Pseudotsuga-hylium), stage VII, the ultimate stage of the
talus succession.
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322. Succession by the accumulation of humus. This is the
characteristic reaction of peat bogs and cypress swamps (oxodia), in which
the accumulation of vegetable matter is enormous. The plant remains
decompose slowly and incompletely under the water, giving rise to the
various humic acids. These possess remarkable antiseptic qualities, and
have an injurious effect upon protoplasm. They affect the absorption of
water by the root-hairs, though this is also influenced by poor aeration. The
same acids are found in practically all inland marshes and swamps, but the
quantity of decomposing vegetation in many is not great enough to produce
an efficient reaction. Formations of this type usually start as freshwater
swamps. The succession is apparently hydrostatic, but no thorough study of
its stages has as yet been made.
323. Succession by modifying atmospheric factors. All layered
formations, forests, thickets, many meadows and wastes, etc., show
reactions of this nature, and are in fact largely or exclusively determined by
them. The reaction is a complex one, though it is clear that light is the most
efficient of the modified factors, and that humidity, temperature, and wind,
while strongly affected, play subordinate parts. In normal successions, the
effect of shade, i. e., diffuse light, enters with the appearance of bushes or
shrubs, and becomes more and more pronounced in the ultimate forest
stages. The reaction is exerted chiefly by the facies, but the effect of this is
to cause increasing diffuseness in each successively lower layer, in direct
ratio with the increased branching and leaf expansion of the plants in the
layer just above. In the ultimate stage of many forests, especially where the
facies are reduced to one, the reaction of the primary layer is so intense as
to preclude all undergrowth. Anomalous successions often owe their origin
to the fact that certain trees react in such a way as to cause conditions in
which they produce seedlings with increasing difficulty, and thus offer a
field favorable to the ecesis of those species capable of enduring the dense
shade. Successions of this kind are almost invariably mesostatic, as it is
altogether exceptional that layered formations are either xerophytic or
hydrophytic.
LAWS OF SUCCESSION
characteristic reaction of peat bogs and cypress swamps (oxodia), in which
the accumulation of vegetable matter is enormous. The plant remains
decompose slowly and incompletely under the water, giving rise to the
various humic acids. These possess remarkable antiseptic qualities, and
have an injurious effect upon protoplasm. They affect the absorption of
water by the root-hairs, though this is also influenced by poor aeration. The
same acids are found in practically all inland marshes and swamps, but the
quantity of decomposing vegetation in many is not great enough to produce
an efficient reaction. Formations of this type usually start as freshwater
swamps. The succession is apparently hydrostatic, but no thorough study of
its stages has as yet been made.
323. Succession by modifying atmospheric factors. All layered
formations, forests, thickets, many meadows and wastes, etc., show
reactions of this nature, and are in fact largely or exclusively determined by
them. The reaction is a complex one, though it is clear that light is the most
efficient of the modified factors, and that humidity, temperature, and wind,
while strongly affected, play subordinate parts. In normal successions, the
effect of shade, i. e., diffuse light, enters with the appearance of bushes or
shrubs, and becomes more and more pronounced in the ultimate forest
stages. The reaction is exerted chiefly by the facies, but the effect of this is
to cause increasing diffuseness in each successively lower layer, in direct
ratio with the increased branching and leaf expansion of the plants in the
layer just above. In the ultimate stage of many forests, especially where the
facies are reduced to one, the reaction of the primary layer is so intense as
to preclude all undergrowth. Anomalous successions often owe their origin
to the fact that certain trees react in such a way as to cause conditions in
which they produce seedlings with increasing difficulty, and thus offer a
field favorable to the ecesis of those species capable of enduring the dense
shade. Successions of this kind are almost invariably mesostatic, as it is
altogether exceptional that layered formations are either xerophytic or
hydrophytic.
LAWS OF SUCCESSION
Page 341
324. The investigation of succession has so far been neither sufficiently
thorough nor systematic to permit the postulation of definite laws. Enough
has been done, however, to warrant the formulation of a number of rules,
which apply to the successions studied, and afford a convenient method for
the critical investigation of all successions upon the basis of initial causes,
and reactions. Warming has already brought together a few such rules, and
an attempt is here made to reduce the phenomena of succession, including
its causes and effects, to a tentative system. At present it is difficult to make
a thoroughly satisfactory classification of such rules, and they are here
arranged in general conformity with the procedure in succession.
I. Causation. The initial cause of a succession is the formation or
appearance of a new habitat, or the efficient change of an existing
one.
II. Reaction. Each stage reacts upon the habitat in such a way as to
produce physical conditions more or less unfavorable to its
permanence, but advantageous to the invaders of the next stage.
III. Proximity and mobility.
(1) The pioneers of a succession are those species nearest
at hand that are the most mobile.
(2) The number of migrants from any formation into a
habitat varies inversely as the square of the distance.
(3) The pioneer species are regularly derived from
different formations, as the latter nearly always contain
permobile species capable of effective ecesis.
(4) The plants of the initial stages are normally algae and
fungi, with minute spores, composites, and grasses,
which possess permobile fruits, or ruderal plants, on
account of their great seed production.
IV. Ecesis.
(1) All the migrants into a new, denuded, or greatly
modified habitat are sorted by ecesis into three groups:
thorough nor systematic to permit the postulation of definite laws. Enough
has been done, however, to warrant the formulation of a number of rules,
which apply to the successions studied, and afford a convenient method for
the critical investigation of all successions upon the basis of initial causes,
and reactions. Warming has already brought together a few such rules, and
an attempt is here made to reduce the phenomena of succession, including
its causes and effects, to a tentative system. At present it is difficult to make
a thoroughly satisfactory classification of such rules, and they are here
arranged in general conformity with the procedure in succession.
I. Causation. The initial cause of a succession is the formation or
appearance of a new habitat, or the efficient change of an existing
one.
II. Reaction. Each stage reacts upon the habitat in such a way as to
produce physical conditions more or less unfavorable to its
permanence, but advantageous to the invaders of the next stage.
III. Proximity and mobility.
(1) The pioneers of a succession are those species nearest
at hand that are the most mobile.
(2) The number of migrants from any formation into a
habitat varies inversely as the square of the distance.
(3) The pioneer species are regularly derived from
different formations, as the latter nearly always contain
permobile species capable of effective ecesis.
(4) The plants of the initial stages are normally algae and
fungi, with minute spores, composites, and grasses,
which possess permobile fruits, or ruderal plants, on
account of their great seed production.
IV. Ecesis.
(1) All the migrants into a new, denuded, or greatly
modified habitat are sorted by ecesis into three groups:
Page 342
(1) those that are unable to germinate or grow, and
soon die; (2) those that grow normally under the
conditions present; (3) those that pass through one or
more of the earlier stages in a dormant state to appear
at a later stage of the succession.
(2) Wherever ruderal vegetation is present, it contributes
a large number of the pioneer species of each
succession, on account of the thorough ecesis. In other
regions this part is played by subruderal native species.
(3) Annuals and biennials are characteristic of the early
stages of secondary successions, on account of their
great seed production and ready ecesis.
(4) In layered formations, heliophytes appear before
sciophytes; they ultimately yield to the latter, except
where they are able to maintain a position in the
primary layer.
(5) Excessive seed production and slight mobility lead to
the imperfect ecesis of individuals in dense stands, and
in consequence usually produce great instability.
(6) Each pioneer produces about itself a tiny area of
ecesis and stabilization for its own offspring, for the
disseminules of its fellows, or of invaders.
(7) Species propagating by offshoots, or producing
relatively immobile disseminules in small number,
usually show effective ecesis, as the offspring appear
within the area of the reaction of the parent forms.
V. Stabilization.
(1) Stabilization is the universal tendency of vegetation.
(2) The ultimate stage of a succession is determined by
the dominant vegetation of the region. Lichen
formations are often ultimate in polar and niveal
zones; grassland is the final vegetation for plains and
alpine stretches, and for much prairie, while forest is
soon die; (2) those that grow normally under the
conditions present; (3) those that pass through one or
more of the earlier stages in a dormant state to appear
at a later stage of the succession.
(2) Wherever ruderal vegetation is present, it contributes
a large number of the pioneer species of each
succession, on account of the thorough ecesis. In other
regions this part is played by subruderal native species.
(3) Annuals and biennials are characteristic of the early
stages of secondary successions, on account of their
great seed production and ready ecesis.
(4) In layered formations, heliophytes appear before
sciophytes; they ultimately yield to the latter, except
where they are able to maintain a position in the
primary layer.
(5) Excessive seed production and slight mobility lead to
the imperfect ecesis of individuals in dense stands, and
in consequence usually produce great instability.
(6) Each pioneer produces about itself a tiny area of
ecesis and stabilization for its own offspring, for the
disseminules of its fellows, or of invaders.
(7) Species propagating by offshoots, or producing
relatively immobile disseminules in small number,
usually show effective ecesis, as the offspring appear
within the area of the reaction of the parent forms.
V. Stabilization.
(1) Stabilization is the universal tendency of vegetation.
(2) The ultimate stage of a succession is determined by
the dominant vegetation of the region. Lichen
formations are often ultimate in polar and niveal
zones; grassland is the final vegetation for plains and
alpine stretches, and for much prairie, while forest is
Page 343
the last stage for mesophytic midlands and lowlands,
as well as for subalpine regions.
(3) Grassland or forest is the usual terminus of a
succession; they predominate in lands
physiographically mature.
(4) The limit of a succession is determined in large part
by the progressive increase in occupation, which
makes the entrance of invaders more and more
difficult.
(5) Stabilization proceeds radiately from the pioneer
plants or masses. The movement of offshoots is away
from the parent mass, and the chances of ecesis are
greatest near its edges, in a narrow area in which the
reaction is still felt, and the occupation is not
exclusive.
VI. General laws.
(1) The stages, or formations, of a succession are
distinguished as initial (prodophytia), intermediate
(ptenophytia), and ultimate (aiphytia).
(2) Initial formations are open, ultimate formations are
closed.
(3) The number of species is small in the initial stages; it
attains a maximum in intermediate stages; and again
decreases in the ultimate formation, on account of the
dominance of a few species.
(4) The normal sequence of vegetation forms in
succession is: (1) algae, fungi, mosses; (2) annuals and
biennials; (3) perennial herbs; (4) bushes and shrubs;
(5) trees.
(5) The number of species and of individuals in each
stage increases constantly up to a maximum, after
which it gradually decreases before the forms of the
as well as for subalpine regions.
(3) Grassland or forest is the usual terminus of a
succession; they predominate in lands
physiographically mature.
(4) The limit of a succession is determined in large part
by the progressive increase in occupation, which
makes the entrance of invaders more and more
difficult.
(5) Stabilization proceeds radiately from the pioneer
plants or masses. The movement of offshoots is away
from the parent mass, and the chances of ecesis are
greatest near its edges, in a narrow area in which the
reaction is still felt, and the occupation is not
exclusive.
VI. General laws.
(1) The stages, or formations, of a succession are
distinguished as initial (prodophytia), intermediate
(ptenophytia), and ultimate (aiphytia).
(2) Initial formations are open, ultimate formations are
closed.
(3) The number of species is small in the initial stages; it
attains a maximum in intermediate stages; and again
decreases in the ultimate formation, on account of the
dominance of a few species.
(4) The normal sequence of vegetation forms in
succession is: (1) algae, fungi, mosses; (2) annuals and
biennials; (3) perennial herbs; (4) bushes and shrubs;
(5) trees.
(5) The number of species and of individuals in each
stage increases constantly up to a maximum, after
which it gradually decreases before the forms of the
Page 344
next stage. The interval between two maxima is
occupied by a mixed formation.
(6) A secondary succession does not begin with the initial
stage of the primary one which it replaces, but usually
at a much later stage.
(7) At present, successions are generally mesotropic,
grassland and forest being the ultimate stages, though
many are xerostatic or hydrostatic. If erosion continue
until the sea level is reached, the ultimate vegetation of
the globe will be hydrophytic. Should the heat of the
sun decrease greatly before this time, the last
vegetation will be xerophytic, i. e., crymophytic.
(8) The operation of succession was essentially the same
during the geological past as it is to-day. From the
nature of their vegetation forms, the record deals
largely with the ultimate stages of such successions.
CLASSIFICATION AND NOMENCLATURE
325. Basis. New or denuded habitats arise the world over by the
operation of the same or similar causes, and they are revegetated in
consequence of the same reactions. Similar habitats produce similar
successions. The vegetation forms and their sequence are usually identical,
and the genera are frequently the same, or corresponding in regions not
entirely unrelated. The species are derived from the adjacent vegetation,
and, except in alpine and coast regions, are normally different. The primary
groups of successions are determined by essential identity of habitat or
cause, e. g., aeolian successions, erosion successions, burn successions, etc.
When they have been more generally investigated, it will be possible to
distinguish subordinate groups of successions, in which the degree of
relationship is indicated by the similarity of vegetation forms, the number
of common genera, etc. For example, burn successions in the Ural and in
the Rocky mountains show almost complete similarity in the matter of
vegetation forms and their sequence, and have the majority of their genera
in common. A natural classification of successions will divide them first of
occupied by a mixed formation.
(6) A secondary succession does not begin with the initial
stage of the primary one which it replaces, but usually
at a much later stage.
(7) At present, successions are generally mesotropic,
grassland and forest being the ultimate stages, though
many are xerostatic or hydrostatic. If erosion continue
until the sea level is reached, the ultimate vegetation of
the globe will be hydrophytic. Should the heat of the
sun decrease greatly before this time, the last
vegetation will be xerophytic, i. e., crymophytic.
(8) The operation of succession was essentially the same
during the geological past as it is to-day. From the
nature of their vegetation forms, the record deals
largely with the ultimate stages of such successions.
CLASSIFICATION AND NOMENCLATURE
325. Basis. New or denuded habitats arise the world over by the
operation of the same or similar causes, and they are revegetated in
consequence of the same reactions. Similar habitats produce similar
successions. The vegetation forms and their sequence are usually identical,
and the genera are frequently the same, or corresponding in regions not
entirely unrelated. The species are derived from the adjacent vegetation,
and, except in alpine and coast regions, are normally different. The primary
groups of successions are determined by essential identity of habitat or
cause, e. g., aeolian successions, erosion successions, burn successions, etc.
When they have been more generally investigated, it will be possible to
distinguish subordinate groups of successions, in which the degree of
relationship is indicated by the similarity of vegetation forms, the number
of common genera, etc. For example, burn successions in the Ural and in
the Rocky mountains show almost complete similarity in the matter of
vegetation forms and their sequence, and have the majority of their genera
in common. A natural classification of successions will divide them first of
Page 345
all into normal and anomalous. The former fall into two classes, primary
and secondary, and these are subdivided into a number of groups, based
upon the cause which initiates the succession.
Fig. 69. Aspen forest formation (Populus-hylium), the typical stage of burn successions in the Rocky
mountains; it is sometimes an anomalous stage in primary successions, interpolated in place of the
thicket formation.
326. Nomenclature. The need of short distinctive names of international
value for plant formations is obvious; it has become imperative that
successions also should be distinguished critically and designated clearly.
From the very nature of the case, it is impossible to designate each
formation or succession by a single Greek or Latin term, as habitats of the
same character will show in different parts of the world a vegetation
taxonomically very different. It may some day be possible to use a binomial
or trinomial for this purpose, somewhat after the fashion of taxonomy, in
and secondary, and these are subdivided into a number of groups, based
upon the cause which initiates the succession.
Fig. 69. Aspen forest formation (Populus-hylium), the typical stage of burn successions in the Rocky
mountains; it is sometimes an anomalous stage in primary successions, interpolated in place of the
thicket formation.
326. Nomenclature. The need of short distinctive names of international
value for plant formations is obvious; it has become imperative that
successions also should be distinguished critically and designated clearly.
From the very nature of the case, it is impossible to designate each
formation or succession by a single Greek or Latin term, as habitats of the
same character will show in different parts of the world a vegetation
taxonomically very different. It may some day be possible to use a binomial
or trinomial for this purpose, somewhat after the fashion of taxonomy, in
Page 346
which the habitat name will represent the generic idea as applied to
formations, and a term drawn from the floristic impress the specific idea.
Such an attempt would be futile or valueless at the present time; it could not
possibly meet with success until there is more uniformity in the concept of
the formation, and until there has been much accurate and thorough
investigation of actual formations, a task as yet barely begun. At present, it
seems most feasible as well as scientific to designate all formations
occupying similar habitats by a name drawn from the character of the latter,
such as a meadow formation, poium, a forest formation, hylium, a desert
formation, eremium, etc. A particular formation is best designated by using
the generic name of one or two of its most important species in conjunction
with its habitat term, as Spartina-Elymus-poium, Picea-Pinus-hylium,
Cereus-Yucca-eremium, etc. Apparently a somewhat similar nomenclature is
adapted to successions. The cause which produces a new habitat may well
furnish the basis for the name of the general groups of successions, as
pyrium (literally, a place or a habitat burned over), a burn succession,
tribium, an erosion succession, etc. A burn succession consists of a
sequence of certain formations in one part of the world, and of a series of
quite different ones, floristically, in another. A particular burn succession
should be designated by using the names of a characteristic facies of the
initial and ultimate stages in connection with the general term, e. g., Bryum-
Picea-pyrium, etc. A trinomial constructed in this way represents the
desirable mean between definition and brevity. Greater definiteness is
possible only at the expense of brevity, while to shorten the name would
entirely destroy its precision. The following classification of successions is
proposed, based upon the plan outlined above. The termination -ium (εῖον)
has been used throughout in the construction of names for successions,
largely for reasons of euphony. If it should become desirable to distinguish
the names of formations and successions by the termination, the locative
suffix -on (-ών) should be used for the latter. The terms given below would
then be hypson, rhyson, hedon, sphyron, prochoson, pnoon, pagon, tribon,
clyson, repon, olisthon, xerasion, theron, broton, pyron, ecballon, camnon,
ocheton, ardon.
I. Normal successions: cyriodochae (κύριος, regular, δοχή, ἡ,
succession)
formations, and a term drawn from the floristic impress the specific idea.
Such an attempt would be futile or valueless at the present time; it could not
possibly meet with success until there is more uniformity in the concept of
the formation, and until there has been much accurate and thorough
investigation of actual formations, a task as yet barely begun. At present, it
seems most feasible as well as scientific to designate all formations
occupying similar habitats by a name drawn from the character of the latter,
such as a meadow formation, poium, a forest formation, hylium, a desert
formation, eremium, etc. A particular formation is best designated by using
the generic name of one or two of its most important species in conjunction
with its habitat term, as Spartina-Elymus-poium, Picea-Pinus-hylium,
Cereus-Yucca-eremium, etc. Apparently a somewhat similar nomenclature is
adapted to successions. The cause which produces a new habitat may well
furnish the basis for the name of the general groups of successions, as
pyrium (literally, a place or a habitat burned over), a burn succession,
tribium, an erosion succession, etc. A burn succession consists of a
sequence of certain formations in one part of the world, and of a series of
quite different ones, floristically, in another. A particular burn succession
should be designated by using the names of a characteristic facies of the
initial and ultimate stages in connection with the general term, e. g., Bryum-
Picea-pyrium, etc. A trinomial constructed in this way represents the
desirable mean between definition and brevity. Greater definiteness is
possible only at the expense of brevity, while to shorten the name would
entirely destroy its precision. The following classification of successions is
proposed, based upon the plan outlined above. The termination -ium (εῖον)
has been used throughout in the construction of names for successions,
largely for reasons of euphony. If it should become desirable to distinguish
the names of formations and successions by the termination, the locative
suffix -on (-ών) should be used for the latter. The terms given below would
then be hypson, rhyson, hedon, sphyron, prochoson, pnoon, pagon, tribon,
clyson, repon, olisthon, xerasion, theron, broton, pyron, ecballon, camnon,
ocheton, ardon.
I. Normal successions: cyriodochae (κύριος, regular, δοχή, ἡ,
succession)
Page 347
a. Primary successions: protodochae (πρῶτος, first,
primary)
1. By elevation: hypsium (ὔψος, το, height, elevation, -εῖον,
place)
2. By volcanic action: rhysium (ῥυσίς ἡ, flowing, especially
of fire)
3. In residuary soils: hedium (ἔδος, τό, a sitting base)
4. In colluvial soils: sphyrium (σφύρον, τό, ankle, talus)
5. In alluvial soils: prochosium (πρόχωσις, ἡ, a deposition
of mud)
6. In aeolian soils: pnoium (πνοή, ἡ, blowing, blast)
7. In glacial soils: pagium (πάγος, ὁ, that which becomes
solid, i. e., a glacier)
b. Secondary successions: hepodochae (ἕπω, to follow)
8. In eroded soils: tribium (τρίβω, wear or rub away)
9. In flooded soils: clysium (κλύσις, ὁ, a drenching,
flooding)
10. By subsidence: repium (ῥέπω, incline downwards, sink)
11. In landslips: olisthium (ὄλισθος, ὁ, slip)
12. In drained and dried out soils: xerasium (ξηρασία, ἡ,
drought)
13. By animal agencies: therium (θήρ, ὁ, wild animal)
14. By human agency: brotium (βροτός, ὁ, a mortal)
a. Burns: pyrium (πῦρ, τό, fire)
b. Lumbering: ecballium (ἐκβάλλω, cut down forests)
c. Cultivation: camnium (κάμνω, cultivate)
d. Drainage: ochetium (ὀχετός, ὁ, drain)
e. Irrigation: ardium (ἄρδω, irrigate)
primary)
1. By elevation: hypsium (ὔψος, το, height, elevation, -εῖον,
place)
2. By volcanic action: rhysium (ῥυσίς ἡ, flowing, especially
of fire)
3. In residuary soils: hedium (ἔδος, τό, a sitting base)
4. In colluvial soils: sphyrium (σφύρον, τό, ankle, talus)
5. In alluvial soils: prochosium (πρόχωσις, ἡ, a deposition
of mud)
6. In aeolian soils: pnoium (πνοή, ἡ, blowing, blast)
7. In glacial soils: pagium (πάγος, ὁ, that which becomes
solid, i. e., a glacier)
b. Secondary successions: hepodochae (ἕπω, to follow)
8. In eroded soils: tribium (τρίβω, wear or rub away)
9. In flooded soils: clysium (κλύσις, ὁ, a drenching,
flooding)
10. By subsidence: repium (ῥέπω, incline downwards, sink)
11. In landslips: olisthium (ὄλισθος, ὁ, slip)
12. In drained and dried out soils: xerasium (ξηρασία, ἡ,
drought)
13. By animal agencies: therium (θήρ, ὁ, wild animal)
14. By human agency: brotium (βροτός, ὁ, a mortal)
a. Burns: pyrium (πῦρ, τό, fire)
b. Lumbering: ecballium (ἐκβάλλω, cut down forests)
c. Cultivation: camnium (κάμνω, cultivate)
d. Drainage: ochetium (ὀχετός, ὁ, drain)
e. Irrigation: ardium (ἄρδω, irrigate)
Page 348
II. Anomalous successions: xenodochae (ξένος, strange, unusual)
327. Illustrations. The following series will illustrate the application of
this system of nomenclature to particular successions, and their stages, or
formations.
Thlaspi-Picea-sphyrium: pennycress-spruce talus succession
Thlaspi-Eriogonum-chalicium: pennycress-eriogonum gravel slide
formation
Elymus-Gilia-chalicium: wildrye-gilia half gravel slide formation
Quercus-Holodiscus-driodium: oak-fringewood dry thicket formation
Pinus-xerohylium: pine dry forest formation
Picea-Pseudotsuga-hylium: spruce-balsam forest formation
Bryum-Picea-pyrium: moss-spruce burn succession
Bryum-telmatium: moss meadow formation
Aster-Chamaenerium-poium: aster-fireweed meadow formation
Deschampsia-Carex-poium: hairgrass-sedge meadow formation
Salix-Betula-helodrium: willow-birch meadow thicket formation
Populus-hylium: aspen forest formation
Picea-hylium: spruce forest formation
Lecanora-Carex-hedium: lichen-carex residuary succession
Lecanora-Gyrophora-petrium: crustose lichen rock formation
Parmelia-Cetraria-chalicium: foliose lichen gravel slide formation
Paronychia-Silene-chalicium: nailwort-campion gravel slide formation
Carex-Campanula-coryphium: sedge-bluebell alpine meadow formation
Eragrostis-Helianthus-xerasium: eragrostis-sunflower drainage succession
Eragrostis-Polygonum-telmatium: eragrostis-heartsease wet meadow
formation
Helianthus-Ambrosia-chledium: sunflower-ragweed waste formation
INVESTIGATION OF SUCCESSION
328. General rules. The study of succession must proceed along two
fundamental lines of inquiry: it is necessary to investigate quantitatively the
physical factors of the initial stages and the reactions produced by the
327. Illustrations. The following series will illustrate the application of
this system of nomenclature to particular successions, and their stages, or
formations.
Thlaspi-Picea-sphyrium: pennycress-spruce talus succession
Thlaspi-Eriogonum-chalicium: pennycress-eriogonum gravel slide
formation
Elymus-Gilia-chalicium: wildrye-gilia half gravel slide formation
Quercus-Holodiscus-driodium: oak-fringewood dry thicket formation
Pinus-xerohylium: pine dry forest formation
Picea-Pseudotsuga-hylium: spruce-balsam forest formation
Bryum-Picea-pyrium: moss-spruce burn succession
Bryum-telmatium: moss meadow formation
Aster-Chamaenerium-poium: aster-fireweed meadow formation
Deschampsia-Carex-poium: hairgrass-sedge meadow formation
Salix-Betula-helodrium: willow-birch meadow thicket formation
Populus-hylium: aspen forest formation
Picea-hylium: spruce forest formation
Lecanora-Carex-hedium: lichen-carex residuary succession
Lecanora-Gyrophora-petrium: crustose lichen rock formation
Parmelia-Cetraria-chalicium: foliose lichen gravel slide formation
Paronychia-Silene-chalicium: nailwort-campion gravel slide formation
Carex-Campanula-coryphium: sedge-bluebell alpine meadow formation
Eragrostis-Helianthus-xerasium: eragrostis-sunflower drainage succession
Eragrostis-Polygonum-telmatium: eragrostis-heartsease wet meadow
formation
Helianthus-Ambrosia-chledium: sunflower-ragweed waste formation
INVESTIGATION OF SUCCESSION
328. General rules. The study of succession must proceed along two
fundamental lines of inquiry: it is necessary to investigate quantitatively the
physical factors of the initial stages and the reactions produced by the
Page 349
subsequent stages. This should be done by automatic instruments for
humidity, light, temperature, and wind, in order that a continuous record
may be obtained. Water-content is taken daily or even less frequently, while
soil properties, and physiographic factors, altitude, slope, surface, and
exposure are determined once for all. It is equally needful to determine the
development and structure of each stage with particular reference to the
adjacent formations, to the stage that has just preceded, and the one that is
to follow. For this, the use of the permanent quadrat is imperative, as the
sequence and structure of the stages can be understood only by a minute
study of the shifting and rearrangement of the individuals. Permanent
migration circles are indispensable for tracing movement away from the
pioneer areas by which each stage reaches its maximum. Denuded quadrats
are a material aid in that they furnish important evidence with respect to
migration and ecesis, By means of them, it is possible to determine the
probable development of stages which reach back a decade or more into the
past. In the examination of successions, since cause and effect are so
intimately connected in each reaction, it is especially important that general
and superficial observations upon structure and sequence be replaced by
precise records, and that vague conjectures as to causes and reactions be
supplanted by the accurate determination of the physical factors which
underlie them.
humidity, light, temperature, and wind, in order that a continuous record
may be obtained. Water-content is taken daily or even less frequently, while
soil properties, and physiographic factors, altitude, slope, surface, and
exposure are determined once for all. It is equally needful to determine the
development and structure of each stage with particular reference to the
adjacent formations, to the stage that has just preceded, and the one that is
to follow. For this, the use of the permanent quadrat is imperative, as the
sequence and structure of the stages can be understood only by a minute
study of the shifting and rearrangement of the individuals. Permanent
migration circles are indispensable for tracing movement away from the
pioneer areas by which each stage reaches its maximum. Denuded quadrats
are a material aid in that they furnish important evidence with respect to
migration and ecesis, By means of them, it is possible to determine the
probable development of stages which reach back a decade or more into the
past. In the examination of successions, since cause and effect are so
intimately connected in each reaction, it is especially important that general
and superficial observations upon structure and sequence be replaced by
precise records, and that vague conjectures as to causes and reactions be
supplanted by the accurate determination of the physical factors which
underlie them.
Page 350
Fig. 70. Alternating gravel slides on Mounts Cameron and Palsgrove, from the comparison of which
the initial development of the talus succession has been reconstructed.
329. Method of alternating stages. The period of time through which a
primary succession operates is usually too great to make a complete study
possible within a single lifetime. Secondary successions run their course
much more quickly, and a decade will sometimes suffice for stabilization,
though even here the period is normally longer. The longest and most
complex succession, however, may be accurately studied in a region, where
several examples of the same succession occur in different stages of
development. In the same region, the physical factors of one example of a
particular succession are essentially identical with those of another example
in the same stage. If one is in an initial stage, and the other in an
intermediate condition, the development of the former makes it possible to
reestablish more or less completely the life history of the latter. The same
the initial development of the talus succession has been reconstructed.
329. Method of alternating stages. The period of time through which a
primary succession operates is usually too great to make a complete study
possible within a single lifetime. Secondary successions run their course
much more quickly, and a decade will sometimes suffice for stabilization,
though even here the period is normally longer. The longest and most
complex succession, however, may be accurately studied in a region, where
several examples of the same succession occur in different stages of
development. In the same region, the physical factors of one example of a
particular succession are essentially identical with those of another example
in the same stage. If one is in an initial stage, and the other in an
intermediate condition, the development of the former makes it possible to
reestablish more or less completely the life history of the latter. The same
Page 351
connection may be made between intermediate and ultimate stages, and it is
thus possible to determine with considerable accuracy and within a few
years the sequence of stages in a succession that requires a century or more
for its complete development. In the Rocky mountains, gravel slides (talus
slopes) are remarkably frequent. They occur in all stages of development,
and the alternating slides of different ages furnish an almost perfect record
of this succession. This method lacks the absolute finality which can be
obtained by following a succession in one spot from its inception to final
stabilization, but it is alone feasible for long successions, i. e., those
extending over a score or more of years. When it comes to be universally
recognized as a plain duty for each investigator to leave an exact and
complete record in quadrat maps and quadrat photographs of the stages
studied by him, it will be a simple task for the botanists of one generation to
finish the investigations of succession begun by their predecessors.
330. The relict method of studying succession is next in importance to
the method of alternating areas. The two in fact are supplementary, and
should be used together whenever relicts are present. This method is based
upon the law of successive maxima, viz., the number of species and of
individuals in each stage constantly increases up to a certain maximum,
after which it gradually decreases before the forms of the next stage. In
accordance with this, secondary species usually disappear first, principal
species next, and facies last of all. There are notable exceptions to this,
however, and the safest plan is to use the relict method only when principal
species or facies are left as evidence. An additional reason for this is that
secondary species are more likely to be common to two or more formations.
In the majority of cases, the relict is not modified, and is readily recognized
as belonging properly to a previous stage. This is true of herbs in all the
stages of grassland, and in the initial ones of forest succession. The herbs
and shrubs of earlier stages, which persist in the final forest stages, are
necessarily modified, often in such a degree as to become distinct ecads, or
species. The facies of the stages which precede the ultimate forest are rarely
modified. The application of the relict method, together with the
modification just described, is nicely illustrated by the balsam-spruce
formation at Minnehaha. Of the initial gravel slide stage, the relicts are
Vagnera stellata and Galium boreale, the one modified into Vagnera
leptopetala, and the other into G. boreale hylocolum. The thicket stage is
represented by Holodiscus dumosa, greatly changed in form and branching,
thus possible to determine with considerable accuracy and within a few
years the sequence of stages in a succession that requires a century or more
for its complete development. In the Rocky mountains, gravel slides (talus
slopes) are remarkably frequent. They occur in all stages of development,
and the alternating slides of different ages furnish an almost perfect record
of this succession. This method lacks the absolute finality which can be
obtained by following a succession in one spot from its inception to final
stabilization, but it is alone feasible for long successions, i. e., those
extending over a score or more of years. When it comes to be universally
recognized as a plain duty for each investigator to leave an exact and
complete record in quadrat maps and quadrat photographs of the stages
studied by him, it will be a simple task for the botanists of one generation to
finish the investigations of succession begun by their predecessors.
330. The relict method of studying succession is next in importance to
the method of alternating areas. The two in fact are supplementary, and
should be used together whenever relicts are present. This method is based
upon the law of successive maxima, viz., the number of species and of
individuals in each stage constantly increases up to a certain maximum,
after which it gradually decreases before the forms of the next stage. In
accordance with this, secondary species usually disappear first, principal
species next, and facies last of all. There are notable exceptions to this,
however, and the safest plan is to use the relict method only when principal
species or facies are left as evidence. An additional reason for this is that
secondary species are more likely to be common to two or more formations.
In the majority of cases, the relict is not modified, and is readily recognized
as belonging properly to a previous stage. This is true of herbs in all the
stages of grassland, and in the initial ones of forest succession. The herbs
and shrubs of earlier stages, which persist in the final forest stages, are
necessarily modified, often in such a degree as to become distinct ecads, or
species. The facies of the stages which precede the ultimate forest are rarely
modified. The application of the relict method, together with the
modification just described, is nicely illustrated by the balsam-spruce
formation at Minnehaha. Of the initial gravel slide stage, the relicts are
Vagnera stellata and Galium boreale, the one modified into Vagnera
leptopetala, and the other into G. boreale hylocolum. The thicket stage is
represented by Holodiscus dumosa, greatly changed in form and branching,
Page 352
and in the shape and structure of the leaf. The most striking relict of the
aspen formation is the facies itself, Populus tremuloides. The tall slender
trunks of dead aspens are found in practically every balsam-spruce forest.
In many places, living trees are still found, with small, straggling crowns,
which are vainly trying to outgrow the surrounding conifers. Of the aspen
undergrowth, Rosa sayii, Helianthella parryi, Frasera speciosa, Zygadenus
elegans, Castilleia confusa, Gentiana acuta, and Solidago orophila remain
more or less modified by the diffuse light. It is still a question whether the
aspen stage passes directly into the balsam-spruce forest, or whether a pine
forest intervenes. The presence of both Pinus ponderosa and P. flexilis,
which are scattered more or less uniformly through the formation, furnishes
strong evidence for the latter view.
Fig. 71. Relict spruces and aspens, showing the character of the succession immediately preceding
the burn succession now developing.
aspen formation is the facies itself, Populus tremuloides. The tall slender
trunks of dead aspens are found in practically every balsam-spruce forest.
In many places, living trees are still found, with small, straggling crowns,
which are vainly trying to outgrow the surrounding conifers. Of the aspen
undergrowth, Rosa sayii, Helianthella parryi, Frasera speciosa, Zygadenus
elegans, Castilleia confusa, Gentiana acuta, and Solidago orophila remain
more or less modified by the diffuse light. It is still a question whether the
aspen stage passes directly into the balsam-spruce forest, or whether a pine
forest intervenes. The presence of both Pinus ponderosa and P. flexilis,
which are scattered more or less uniformly through the formation, furnishes
strong evidence for the latter view.
Fig. 71. Relict spruces and aspens, showing the character of the succession immediately preceding
the burn succession now developing.
Page 353
The lifetime of forest and thicket stages of successions is ascertained by
counting the annual rings of the stumps of facies. This is a perfectly feasible
method for many woodland formations where stumps already abound or
where a fire has occurred, and it is but rarely necessary to cut down trees
for this purpose. When trees or shrubs are present as relicts, the same
method is used to determine the length of time taken by the development of
the corresponding stages.
counting the annual rings of the stumps of facies. This is a perfectly feasible
method for many woodland formations where stumps already abound or
where a fire has occurred, and it is but rarely necessary to cut down trees
for this purpose. When trees or shrubs are present as relicts, the same
method is used to determine the length of time taken by the development of
the corresponding stages.
Page 354
THE STRUCTURE OF THE FORMATION
331. Since all the structures exhibited by formations, such as zones,
layers, consocies, etc., are to be referred to zonation or alternation, these
principles are first considered in detail. This, then, constitutes the basis for a
consideration of the structure of a normal formation, with special reference
to the different parts that compose it. The investigation of formational
structure, since the latter is the result of aggregation, invasion, and
succession, is accomplished by instruments, quadrats, etc., in the manner
already indicated under development, and no further discussion of it is
necessary here.
331. Since all the structures exhibited by formations, such as zones,
layers, consocies, etc., are to be referred to zonation or alternation, these
principles are first considered in detail. This, then, constitutes the basis for a
consideration of the structure of a normal formation, with special reference
to the different parts that compose it. The investigation of formational
structure, since the latter is the result of aggregation, invasion, and
succession, is accomplished by instruments, quadrats, etc., in the manner
already indicated under development, and no further discussion of it is
necessary here.
Page 355
ZONATION
332. Concept. The recognition of vegetation zones dates from
Tournefort[40], who found that, while the plants of Armenia occupied the
foot of Mount Ararat, the vegetation of the slopes above contained many
species of southern Europe. Still higher appeared a flora similar to that of
Sweden, and on the summit grew arctic plants, such as those of Lapland.
As the historical summary shows, the concept of zonation is the oldest in
phytogeography. Notwithstanding this, it has never been clearly defined,
nor has there been any detailed investigation of the phenomenon itself, or of
the causes which produce it. Zones are so common, and often so clearly
marked, that they invite study, but no serious attempt has heretofore been
made to analyze zonation, or to formulate a definite method of investigating
it. Zonation is the practically universal response of plants to the quantitative
distribution of physical factors in nature. In almost all habitats, one or more
of the physical factors present decreases gradually in passing away from the
point of greatest intensity. The result is that the plants of the habitat arrange
themselves in belts about this point, their position being determined by their
relation to the factor concerned. Close investigation will show that there is
hardly a formation that is entirely without zonation, though in many cases
the zones are incomplete or obscure for various reasons. Zonation is as
characteristic of vegetation as a whole as it is of its unit, the formation, a
fact long ago recognized in temperature zones. A continental climate,
however, often results in the interruption of these, with the consequence that
these belts of vegetation are not always continuous.
CAUSES OF ZONATION
333. Growth. The causes that produce zones are either biological or
physical: the first have to do with some characteristic of the plant, the
second with the physical features of the habitat. Biological causes arise
from the method of growth, from the manner of dissemination, or from the
reaction of the species upon the habitat. The formation of circles as a result
of radial growth is a well-known occurrence with certain plants, but it is
332. Concept. The recognition of vegetation zones dates from
Tournefort[40], who found that, while the plants of Armenia occupied the
foot of Mount Ararat, the vegetation of the slopes above contained many
species of southern Europe. Still higher appeared a flora similar to that of
Sweden, and on the summit grew arctic plants, such as those of Lapland.
As the historical summary shows, the concept of zonation is the oldest in
phytogeography. Notwithstanding this, it has never been clearly defined,
nor has there been any detailed investigation of the phenomenon itself, or of
the causes which produce it. Zones are so common, and often so clearly
marked, that they invite study, but no serious attempt has heretofore been
made to analyze zonation, or to formulate a definite method of investigating
it. Zonation is the practically universal response of plants to the quantitative
distribution of physical factors in nature. In almost all habitats, one or more
of the physical factors present decreases gradually in passing away from the
point of greatest intensity. The result is that the plants of the habitat arrange
themselves in belts about this point, their position being determined by their
relation to the factor concerned. Close investigation will show that there is
hardly a formation that is entirely without zonation, though in many cases
the zones are incomplete or obscure for various reasons. Zonation is as
characteristic of vegetation as a whole as it is of its unit, the formation, a
fact long ago recognized in temperature zones. A continental climate,
however, often results in the interruption of these, with the consequence that
these belts of vegetation are not always continuous.
CAUSES OF ZONATION
333. Growth. The causes that produce zones are either biological or
physical: the first have to do with some characteristic of the plant, the
second with the physical features of the habitat. Biological causes arise
from the method of growth, from the manner of dissemination, or from the
reaction of the species upon the habitat. The formation of circles as a result
of radial growth is a well-known occurrence with certain plants, but it is
Page 356
much more common than is supposed. In the case of agarics, this
phenomenon has long been known under the name of “fairy-rings.” It is
found in a large number of moulds, and is characteristic of early stages of
the mycelium of the powdery mildews. It occurs in nearly all maculicole
fungi, and is exhibited by certain xylogenous fungi, such as
Hysterographium. Among the foliose lichens, it is a common occurrence
with the rock forms of Parmelia, Placodium, Physcia, and Lecanora, and
with the earth forms of Parmelia and Peltigera. The thalloid liverworts
show a similar radial growth. The flowering plants, and many mosses also,
furnish good examples of this sort of growth in those species which
simulate the form of the mycelium or thallus. These are the species that
form mats, turfs, or carpets. Alpine mat formers, such as Silene acaulis,
Paronychia pulvinata, Arenaria sajanesis, etc., are typical examples.
Xerophytic, turf-forming species of Muhlenbergia, Sporobolus, Bouteloua,
Festuca, Poa, and other grasses form striking ring-like mats, while creeping
species of Euphorbia, Portulaca, Amarantus, etc., produce circular areas.
Rosettes, bunch-grasses, and many ordinary rootstalk plants spread rapidly
by runners and rhizomes. The direction of growth is often indeterminate in
these also, and is in consequence more or less bilateral or unilateral. Growth
results in zonation only when the older central portions of the individual or
mass die away, leaving an ever-widening belt of younger plants or parts.
This phenomenon is doubtless due in part to the greater age of the central
portion, but seems to arise chiefly from the demands made by the young
and actively growing parts upon the water of the soil. There may possibly
be an exhaustion of nutritive content, as in the case of the fungi, but this
seems improbable for the reason that young plants of the same and other
species thrive in these areas. It must not be inferred that these miniature
growth zones increase in size until they pass into zones of formations.
Growth contributes its share to the production of these, but there is no
genetic connection between a tiny plant zone and a zone of vegetation.
Radial and bilateral growth play an important part in formational zones in
so far as they are related to migration. The growth of the runner or rhizome
itself is a very effective means of dissemination, while the seeding of the
plants thus carried away from the central mass is most effective at the edge
of the newly occupied area. This holds with equal force for plants with a
mycelium or a thallus. The circular area becomes larger year by year.
Sooner or later, the younger, more vigorous, and more completely occupied
phenomenon has long been known under the name of “fairy-rings.” It is
found in a large number of moulds, and is characteristic of early stages of
the mycelium of the powdery mildews. It occurs in nearly all maculicole
fungi, and is exhibited by certain xylogenous fungi, such as
Hysterographium. Among the foliose lichens, it is a common occurrence
with the rock forms of Parmelia, Placodium, Physcia, and Lecanora, and
with the earth forms of Parmelia and Peltigera. The thalloid liverworts
show a similar radial growth. The flowering plants, and many mosses also,
furnish good examples of this sort of growth in those species which
simulate the form of the mycelium or thallus. These are the species that
form mats, turfs, or carpets. Alpine mat formers, such as Silene acaulis,
Paronychia pulvinata, Arenaria sajanesis, etc., are typical examples.
Xerophytic, turf-forming species of Muhlenbergia, Sporobolus, Bouteloua,
Festuca, Poa, and other grasses form striking ring-like mats, while creeping
species of Euphorbia, Portulaca, Amarantus, etc., produce circular areas.
Rosettes, bunch-grasses, and many ordinary rootstalk plants spread rapidly
by runners and rhizomes. The direction of growth is often indeterminate in
these also, and is in consequence more or less bilateral or unilateral. Growth
results in zonation only when the older central portions of the individual or
mass die away, leaving an ever-widening belt of younger plants or parts.
This phenomenon is doubtless due in part to the greater age of the central
portion, but seems to arise chiefly from the demands made by the young
and actively growing parts upon the water of the soil. There may possibly
be an exhaustion of nutritive content, as in the case of the fungi, but this
seems improbable for the reason that young plants of the same and other
species thrive in these areas. It must not be inferred that these miniature
growth zones increase in size until they pass into zones of formations.
Growth contributes its share to the production of these, but there is no
genetic connection between a tiny plant zone and a zone of vegetation.
Radial and bilateral growth play an important part in formational zones in
so far as they are related to migration. The growth of the runner or rhizome
itself is a very effective means of dissemination, while the seeding of the
plants thus carried away from the central mass is most effective at the edge
of the newly occupied area. This holds with equal force for plants with a
mycelium or a thallus. The circular area becomes larger year by year.
Sooner or later, the younger, more vigorous, and more completely occupied
Page 357
circumference passes into a more or less complete zone. This will result
from the reaction of the central individuals upon the habitat, so that they are
readily displaced by invaders, or from their increasing senility and dying
out, or from the invasion of forms which seed more abundantly and
successfully. This result will only be the more marked if the radiating
migrants reach a belt of ground especially favorable to their ecesis. In this
connection it must be carefully noted that vegetation pressure, before which
weaker plants are generally supposed to flee, or by which they are thought
to be forced out into less desirable situations, is little more than a fanciful
term for radial growth and migration. It has been shown under invasion that
disseminules move into vegetation masses, as well as away from them, the
outward movement alone being conspicuous, because it is only at the
margin and beyond that they find the necessary water and light for growth.
334. Reactions. Certain reactions of plants upon habitats produce
zonation. The zones of fungi are doubtless caused by the exhaustion of the
organic matter present, while in lichens and mosses the decrease in nutritive
content has something to do with the disappearance of the central mass. In
the mats of flowering plants, the connection is much less certain. The
reaction of a forest or thicket, or even of a tall herbaceous layer, is an
extremely important factor in the production of zonation. The factor chiefly
concerned here is light. Its intensity is greatest at the edge of the formation
and just below the primary layer; the light becomes increasingly diffuse
toward the center of the forest, and toward the ground. In response to this,
both lateral and vertical zones appear. The former are more or less
incomplete, and are only in part due to differences in illumination. The
vertical zones or layers are characteristic of forest and thickets, and are
caused directly by differences in light intensity.
from the reaction of the central individuals upon the habitat, so that they are
readily displaced by invaders, or from their increasing senility and dying
out, or from the invasion of forms which seed more abundantly and
successfully. This result will only be the more marked if the radiating
migrants reach a belt of ground especially favorable to their ecesis. In this
connection it must be carefully noted that vegetation pressure, before which
weaker plants are generally supposed to flee, or by which they are thought
to be forced out into less desirable situations, is little more than a fanciful
term for radial growth and migration. It has been shown under invasion that
disseminules move into vegetation masses, as well as away from them, the
outward movement alone being conspicuous, because it is only at the
margin and beyond that they find the necessary water and light for growth.
334. Reactions. Certain reactions of plants upon habitats produce
zonation. The zones of fungi are doubtless caused by the exhaustion of the
organic matter present, while in lichens and mosses the decrease in nutritive
content has something to do with the disappearance of the central mass. In
the mats of flowering plants, the connection is much less certain. The
reaction of a forest or thicket, or even of a tall herbaceous layer, is an
extremely important factor in the production of zonation. The factor chiefly
concerned here is light. Its intensity is greatest at the edge of the formation
and just below the primary layer; the light becomes increasingly diffuse
toward the center of the forest, and toward the ground. In response to this,
both lateral and vertical zones appear. The former are more or less
incomplete, and are only in part due to differences in illumination. The
vertical zones or layers are characteristic of forest and thickets, and are
caused directly by differences in light intensity.
Page 358
Fig. 72. Zones of Cyperus erythrorrhizus produced by the recession of the shore-line.
335. Physical factors. The physical causes of zonation are by far the
most important. They arise from differences in temperature, water, and
light. In the large, temperature differences are the most important,
producing the great zones of vegetation. In a particular region or habitat,
variations of water-content and humidity are controlling, while light, as
shown above, is important in the reactions of forest and thicket. Physical
factors produce zonation in a habitat or a series of habitats, when there is
either a gradual and cumulative, or an abrupt change in their intensity.
Gradual, slight changes are typical of single habitats; abrupt, marked
changes of a series of habitats. This modification of a decisive factor tends
to operate in all directions from the place of greatest intensity, producing a
characteristic symmetry of the habitat with reference to the factor
335. Physical factors. The physical causes of zonation are by far the
most important. They arise from differences in temperature, water, and
light. In the large, temperature differences are the most important,
producing the great zones of vegetation. In a particular region or habitat,
variations of water-content and humidity are controlling, while light, as
shown above, is important in the reactions of forest and thicket. Physical
factors produce zonation in a habitat or a series of habitats, when there is
either a gradual and cumulative, or an abrupt change in their intensity.
Gradual, slight changes are typical of single habitats; abrupt, marked
changes of a series of habitats. This modification of a decisive factor tends
to operate in all directions from the place of greatest intensity, producing a
characteristic symmetry of the habitat with reference to the factor
Page 359
concerned. If the area of greatest amount is linear, the shading-out will take
place in two directions, and the symmetry will be bilateral, a condition well
illustrated by rivers. On the other hand, a central intense area will shade out
in all directions, giving rise to radial symmetry, as in ponds, lakes, etc. The
essential connection between these is evident where a stream broadens into
a lake, or the latter is the source of a stream, where a mountain ridge breaks
up into isolated peaks, or where a peninsula or landspit is cut into islands.
The line that connects the points of accumulated or abrupt change in the
symmetry is a stress line or ecotone. Ecotones are well-marked between
formations, particularly where the medium changes; they are less distinct
within formations. It is obvious that an ecotone separates two different
series of zones in the one case, and merely two distinct zones in the other.
Fig. 73. Regional zones on a spur of Pike’s Peak (3,800 m.); the forest consists of Picea engelmannii
and Pinus aristata, the forewold is Salix pseudolapponum, and the grassland, alpine meadow (Carex-
Campanula-coryphium).
place in two directions, and the symmetry will be bilateral, a condition well
illustrated by rivers. On the other hand, a central intense area will shade out
in all directions, giving rise to radial symmetry, as in ponds, lakes, etc. The
essential connection between these is evident where a stream broadens into
a lake, or the latter is the source of a stream, where a mountain ridge breaks
up into isolated peaks, or where a peninsula or landspit is cut into islands.
The line that connects the points of accumulated or abrupt change in the
symmetry is a stress line or ecotone. Ecotones are well-marked between
formations, particularly where the medium changes; they are less distinct
within formations. It is obvious that an ecotone separates two different
series of zones in the one case, and merely two distinct zones in the other.
Fig. 73. Regional zones on a spur of Pike’s Peak (3,800 m.); the forest consists of Picea engelmannii
and Pinus aristata, the forewold is Salix pseudolapponum, and the grassland, alpine meadow (Carex-
Campanula-coryphium).
Page 360
336. Physiographic symmetry. The physical symmetry of a habitat
depends upon the distribution of water in it, and this is profoundly affected
by the soil and the physiography. The influence of precipitation is slight or
lacking, as it is nearly uniform throughout the habitat; the effects of wind
and humidity are more localized. Differences of soil rarely obtain within a
single habitat, though often occurring in a zoned series. The strikingly zonal
structure or arrangement of habitats is nearly always due to differences in
water-content produced by physiographic factors, slope, exposure, surface,
and altitude. The effect of these upon water-content and humidity is
obvious. Wherever appreciable physiographic differences occur, there will
be central areas of excess and deficiency in water-content, between which
there is a symmetrical modification of this factor. Peaks are typical
examples of areas of deficiency, lakes and oceans of areas of excess. When
these areas are extreme and close to each other, the resulting zonation will
be marked; when they are moderate, particularly if they are widely
separated, the zones produced are obscure. Asymmetry of a habitat or a
region practically does not exist. Central areas of excess and deficiency
may be very large and in consequence fail to seem symmetrical, or the
space between them so great that the symmetry is not conspicuous, but they
are everywhere present, acting as foci for the intervening areas.
The response of vegetation to habitat is so intimate that physiographic
symmetry everywhere produces vegetational symmetry, which finds its
ready expression in plant zones. The reaction of vegetation upon habitat
causes biological symmetry, typical of growth zones and light zones. From
these facts it is clear that zonation will be regularly characteristic of the
vegetative covering. The zonal arrangement of formations is usually very
evident; the zones of a formation are often obscured, or, where the latter
occupies a uniform central area of excess or deficiency, they are
rudimentary or lacking, as in shallow ponds. Zones are frequently
imperfect, though rarely entirely absent in new soils, such as talus. They are
rendered obscure in several ways. In the initial stages of a succession, as
well as in the transitions between the various stages, the plant population is
so scattered, so transient, or so dense as to respond not at all to a degree of
symmetry which produces marked zonation in later formations. The
alternation of conspicuous species not only causes great interruption of
zones, but often also completely conceals the zonation of other species,
such as the grasses, which, though of more importance in the formation,
depends upon the distribution of water in it, and this is profoundly affected
by the soil and the physiography. The influence of precipitation is slight or
lacking, as it is nearly uniform throughout the habitat; the effects of wind
and humidity are more localized. Differences of soil rarely obtain within a
single habitat, though often occurring in a zoned series. The strikingly zonal
structure or arrangement of habitats is nearly always due to differences in
water-content produced by physiographic factors, slope, exposure, surface,
and altitude. The effect of these upon water-content and humidity is
obvious. Wherever appreciable physiographic differences occur, there will
be central areas of excess and deficiency in water-content, between which
there is a symmetrical modification of this factor. Peaks are typical
examples of areas of deficiency, lakes and oceans of areas of excess. When
these areas are extreme and close to each other, the resulting zonation will
be marked; when they are moderate, particularly if they are widely
separated, the zones produced are obscure. Asymmetry of a habitat or a
region practically does not exist. Central areas of excess and deficiency
may be very large and in consequence fail to seem symmetrical, or the
space between them so great that the symmetry is not conspicuous, but they
are everywhere present, acting as foci for the intervening areas.
The response of vegetation to habitat is so intimate that physiographic
symmetry everywhere produces vegetational symmetry, which finds its
ready expression in plant zones. The reaction of vegetation upon habitat
causes biological symmetry, typical of growth zones and light zones. From
these facts it is clear that zonation will be regularly characteristic of the
vegetative covering. The zonal arrangement of formations is usually very
evident; the zones of a formation are often obscured, or, where the latter
occupies a uniform central area of excess or deficiency, they are
rudimentary or lacking, as in shallow ponds. Zones are frequently
imperfect, though rarely entirely absent in new soils, such as talus. They are
rendered obscure in several ways. In the initial stages of a succession, as
well as in the transitions between the various stages, the plant population is
so scattered, so transient, or so dense as to respond not at all to a degree of
symmetry which produces marked zonation in later formations. The
alternation of conspicuous species not only causes great interruption of
zones, but often also completely conceals the zonation of other species,
such as the grasses, which, though of more importance in the formation,
Page 361
have a lower habit of growth. Furthermore, the ecotones of one factor may
run at right angles to those of another, and the resulting series of zones
mutually obscure each other. Finally, such a physiographic feature as a hill
may have its symmetry interrupted by ridges or ravines, which deflect the
zones downward or upward, or cause them to disappear altogether, while
the shallows or depths of a pond or lake may have the same effect. An entire
absence of zones, i. e., azonation, is exceptional in vegetation. Almost all
cases that seem to exhibit it may be shown by careful examination to arise
in one of the several ways indicated above.
KINDS OF ZONATION
337. Two kinds of zonation are distinguished with reference to the
direction in which the controlling factor changes. When this is horizontal,
as with water-content and temperature, zonation will be lateral; when it is
vertical, as in the case of light, the zonation is vertical. There exists an
intimate connection between the two in forests, where the secondary layer
of small trees and shrubs is continuous with a belt of trees and shrubs
around the central nucleus, and the lower layers of bushes and herbaceous
plants with similar zones still further out. This connection doubtless arises
from the fact that conditions are unfavorable to the facies, outside of the
nucleus as well as beneath it. Floristically, each layer and its corresponding
zone are distinct, as the one consists of shade, the other of sun species.
Lateral zonation is radial when the habitat or physiographic feature is more
or less circular in form, and it is bilateral, when the latter is elongated or
linear. Vertical zonation is unilateral.
338. Radial zonation is regularly characteristic of elevations and
depressions. From the form of the earth, it reaches its larger expression in
the girdles of vegetation corresponding to the zones of temperature. The
zones of mountain peaks are likewise due largely to temperature, though
humidity is a very important factor also. Mountain zones are normally quite
perfect. The zonation of islands, hills, etc., is due to water-content. In the
former, the zones are usually quite regular and complete; in the latter, they
are often incomplete or obscured. Prairies and steppes are not zoned as
units, but are complexes of more or less zonal hills and ridges. Ponds, lakes,
and seas regularly exhibit complete zones, except in those shallow ponds
run at right angles to those of another, and the resulting series of zones
mutually obscure each other. Finally, such a physiographic feature as a hill
may have its symmetry interrupted by ridges or ravines, which deflect the
zones downward or upward, or cause them to disappear altogether, while
the shallows or depths of a pond or lake may have the same effect. An entire
absence of zones, i. e., azonation, is exceptional in vegetation. Almost all
cases that seem to exhibit it may be shown by careful examination to arise
in one of the several ways indicated above.
KINDS OF ZONATION
337. Two kinds of zonation are distinguished with reference to the
direction in which the controlling factor changes. When this is horizontal,
as with water-content and temperature, zonation will be lateral; when it is
vertical, as in the case of light, the zonation is vertical. There exists an
intimate connection between the two in forests, where the secondary layer
of small trees and shrubs is continuous with a belt of trees and shrubs
around the central nucleus, and the lower layers of bushes and herbaceous
plants with similar zones still further out. This connection doubtless arises
from the fact that conditions are unfavorable to the facies, outside of the
nucleus as well as beneath it. Floristically, each layer and its corresponding
zone are distinct, as the one consists of shade, the other of sun species.
Lateral zonation is radial when the habitat or physiographic feature is more
or less circular in form, and it is bilateral, when the latter is elongated or
linear. Vertical zonation is unilateral.
338. Radial zonation is regularly characteristic of elevations and
depressions. From the form of the earth, it reaches its larger expression in
the girdles of vegetation corresponding to the zones of temperature. The
zones of mountain peaks are likewise due largely to temperature, though
humidity is a very important factor also. Mountain zones are normally quite
perfect. The zonation of islands, hills, etc., is due to water-content. In the
former, the zones are usually quite regular and complete; in the latter, they
are often incomplete or obscured. Prairies and steppes are not zoned as
units, but are complexes of more or less zonal hills and ridges. Ponds, lakes,
and seas regularly exhibit complete zones, except in those shallow ponds
Page 362
where the depth is so slight that what is ordinarily a marginal zone is able to
extend over the entire bottom. The line between an elevation and a
depression, i. e., the edge of the water level, is the most sharply defined of
all ecotones. It separates two series of zones, each of which constitutes a
formation. One of these is regularly hydrophytic, the other is usually
mesophytic. The line between the two can rarely be drawn at the water’s
edge, as this is not a constant, owing to waves, tides, or periodical rise and
fall. There is in consequence a more or less variable transition zone of
amphibious plants, which are, however, to be referred to the hydrophytic
formation. Nearly all forest formations serve as a center about which are
arranged several somewhat complete zones. As a rule, these merge into a
single heterogeneous zone of thickets.
339. Bilateral zonation differs from radial only in as much as it deals
with linear elevations and depressions instead of circular ones. With this
difference, the zones of ranges and ridges correspond exactly to those of
peaks and hills, while the same relation is evident between the zones of
streams, and of lakes and ponds. The ecotones are identical except as to
form; they are linear in the one and circular in the other. Incompleteness is
more frequently found in bilateral zonation, though this is a question of
distance or extent, rather than one of symmetry.
340. Vertical zonation is peculiar in that there is no primary ecotone
present, on either side of which zones arrange themselves with reference to
the factor concerned. This arises from the fact that the controlling factor is
light, which impinges upon the habitat in such manner as to shade out in but
one direction, i. e., downward. Vertical zones appear in bodies of water, on
account of the absorption of light by the water. In a general way, it is
possible to distinguish bottom, plancton, and surface zones, consisting
almost wholly of algae. There is little question that minor zones exist,
especially in lakes and seas, but these await further investigation. The most
characteristic vertical zones occur in forests, where the primary layer of
trees acts as a screen. The density of this screen determines the number of
zones found beneath it. In extreme cases the foliage is so dense that the
light beneath is insufficient even for mosses and lichens. As a rule,
however, there will be one or more zones present. In an ordinary deciduous
forest, the layers below the facies are five or six in number: (1) a secondary
layer of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper
herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a lower
extend over the entire bottom. The line between an elevation and a
depression, i. e., the edge of the water level, is the most sharply defined of
all ecotones. It separates two series of zones, each of which constitutes a
formation. One of these is regularly hydrophytic, the other is usually
mesophytic. The line between the two can rarely be drawn at the water’s
edge, as this is not a constant, owing to waves, tides, or periodical rise and
fall. There is in consequence a more or less variable transition zone of
amphibious plants, which are, however, to be referred to the hydrophytic
formation. Nearly all forest formations serve as a center about which are
arranged several somewhat complete zones. As a rule, these merge into a
single heterogeneous zone of thickets.
339. Bilateral zonation differs from radial only in as much as it deals
with linear elevations and depressions instead of circular ones. With this
difference, the zones of ranges and ridges correspond exactly to those of
peaks and hills, while the same relation is evident between the zones of
streams, and of lakes and ponds. The ecotones are identical except as to
form; they are linear in the one and circular in the other. Incompleteness is
more frequently found in bilateral zonation, though this is a question of
distance or extent, rather than one of symmetry.
340. Vertical zonation is peculiar in that there is no primary ecotone
present, on either side of which zones arrange themselves with reference to
the factor concerned. This arises from the fact that the controlling factor is
light, which impinges upon the habitat in such manner as to shade out in but
one direction, i. e., downward. Vertical zones appear in bodies of water, on
account of the absorption of light by the water. In a general way, it is
possible to distinguish bottom, plancton, and surface zones, consisting
almost wholly of algae. There is little question that minor zones exist,
especially in lakes and seas, but these await further investigation. The most
characteristic vertical zones occur in forests, where the primary layer of
trees acts as a screen. The density of this screen determines the number of
zones found beneath it. In extreme cases the foliage is so dense that the
light beneath is insufficient even for mosses and lichens. As a rule,
however, there will be one or more zones present. In an ordinary deciduous
forest, the layers below the facies are five or six in number: (1) a secondary
layer of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper
herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a lower
Page 363
herbaceous layer, (6) a ground layer of mosses, lichens, other fungi, and
algae. The upper layers are often discontinuous, the lower ones are more
and more continuous. As a forest becomes denser, its layers disappear from
the upper downward, the ground layer always being the last to disappear
because of its ability to grow in very diffuse light. A vertically zoned
formation shows a complex series of reactions. The primary layer
determines the amount of heat, light, water, wind, etc., for the subordinate
layers in general. Each of these layers then further determines the amount
for those below it, the ground layer being subject in some degree to the
control of every layer above it. This accounts probably for the definiteness
and permanence of this layer. The degree to which the lower layers
influence the upper by reacting upon the habitat is not known. It is evident
that this influence must be considerable by virtue of their control of the
water supply in the upper soil strata, by virtue of their transpiration, their
decomposition, etc.
The ecotone between two formations is never a sharp line, but it is an
area of varying width. The edge of this area which is contiguous to one
formation marks the limit for species of the other. Both formations
disappear in this transition zone, but in opposite directions. The overlapping
which produces such zones arises from the fact that the physical factors
tend to approach each other at the line of contact between formations, and
that many species are more or less adjustable to conditions not too
dissimilar.
341. Vegetation zones. As a fundamental expression of progressive
change in the amount of heat and water, zonation is the most important
feature of vegetation. It constitutes the sole basis for the division of
continental as well as insular vegetation. The continent of North America
furnishes striking proof of the truth of this. Conforming to the gradual
decrease of temperature and water-content northward, three primary belts of
vegetation stretch across the continent from east to west. These are forest,
grassland, and polar desert. The first is further divided into the secondary
zones of broad-leaved evergreen, deciduous, and needle-leaved forests. At
right angles to this temperature-water symmetry lies a symmetry due to
water alone, in accordance with which forest belts touch the oceans, but
give way in the interior to grasslands, and these to deserts. It is at once
evident that the mutual interruption of these two series of zones has
produced the primary features of North America vegetation, i. e., tropical
algae. The upper layers are often discontinuous, the lower ones are more
and more continuous. As a forest becomes denser, its layers disappear from
the upper downward, the ground layer always being the last to disappear
because of its ability to grow in very diffuse light. A vertically zoned
formation shows a complex series of reactions. The primary layer
determines the amount of heat, light, water, wind, etc., for the subordinate
layers in general. Each of these layers then further determines the amount
for those below it, the ground layer being subject in some degree to the
control of every layer above it. This accounts probably for the definiteness
and permanence of this layer. The degree to which the lower layers
influence the upper by reacting upon the habitat is not known. It is evident
that this influence must be considerable by virtue of their control of the
water supply in the upper soil strata, by virtue of their transpiration, their
decomposition, etc.
The ecotone between two formations is never a sharp line, but it is an
area of varying width. The edge of this area which is contiguous to one
formation marks the limit for species of the other. Both formations
disappear in this transition zone, but in opposite directions. The overlapping
which produces such zones arises from the fact that the physical factors
tend to approach each other at the line of contact between formations, and
that many species are more or less adjustable to conditions not too
dissimilar.
341. Vegetation zones. As a fundamental expression of progressive
change in the amount of heat and water, zonation is the most important
feature of vegetation. It constitutes the sole basis for the division of
continental as well as insular vegetation. The continent of North America
furnishes striking proof of the truth of this. Conforming to the gradual
decrease of temperature and water-content northward, three primary belts of
vegetation stretch across the continent from east to west. These are forest,
grassland, and polar desert. The first is further divided into the secondary
zones of broad-leaved evergreen, deciduous, and needle-leaved forests. At
right angles to this temperature-water symmetry lies a symmetry due to
water alone, in accordance with which forest belts touch the oceans, but
give way in the interior to grasslands, and these to deserts. It is at once
evident that the mutual interruption of these two series of zones has
produced the primary features of North America vegetation, i. e., tropical
Page 364
forests where heat and water are excessive, deserts where either is
unusually deficient, grassland when one is low, the other moderate, and
deciduous and coniferous forests, where the water-content is as least
moderate and the temperature not too low. Such a simple yet fundamental
division has been modified, however, by the disturbing effect which three
continental mountain systems have had upon humidity and upon
temperature symmetry. The two are intimately interwoven. The lowering of
temperature due to altitude produces the precipitation of the wind-borne
moisture upon those slopes which look toward the quarter from which the
prevailing winds blow. A mountain range thus makes an abrupt change in
the symmetry, and renders impossible the gradual change from forest to
grassland and desert. The Appalachian system is not sufficiently high to
produce a pronounced effect, and forests extend far beyond it into the
interior before passing into prairies and plains. On the other hand, the
influence of the Rocky mountains and the Sierra Nevada is very marked.
The latter rise to a great height relatively near the coast, and condense upon
their western slopes nearly all of the moisture brought from the Pacific. The
Rocky mountains have the same effect upon the much drier winds that blow
from the east, and the two systems in consequence enclose a parched desert.
This series of major zones thus becomes, starting at the east, forest,
grassland, desert, and forest, instead of the more symmetrical series, forest,
grassland, desert, grassland, forest, which would prevail were it not for
these barriers. This actual series of major zones undergoes further
interruption by the action of these mountain systems in deflecting northern
isotherms far to the south. This action is greatest in the high ranges, the
Rocky mountains and the Sierras, and least in the lower Appalachians. Its
result is to carry the polar deserts of the north far southward along the crests
of the mountains, and to extend the boreal coniferous forests much further
south along their slopes. In the Appalachians, this means no more than the
extension of a long tongue of conifers into the mass of deciduous forests,
and the occasional appearance of an isolated peak. In the western ranges, it
produces two symmetrical series of minor mountain zones, forest, alpine
grassland or desert, and forest, to say nothing of the foot-hill and timber-
line zones of thicket.
There seems to be no good reason for distinguishing the zones of
mountains as regions. The term itself is inapplicable, as it has no reference
to zonation, and is used much more frequently as a term of general
unusually deficient, grassland when one is low, the other moderate, and
deciduous and coniferous forests, where the water-content is as least
moderate and the temperature not too low. Such a simple yet fundamental
division has been modified, however, by the disturbing effect which three
continental mountain systems have had upon humidity and upon
temperature symmetry. The two are intimately interwoven. The lowering of
temperature due to altitude produces the precipitation of the wind-borne
moisture upon those slopes which look toward the quarter from which the
prevailing winds blow. A mountain range thus makes an abrupt change in
the symmetry, and renders impossible the gradual change from forest to
grassland and desert. The Appalachian system is not sufficiently high to
produce a pronounced effect, and forests extend far beyond it into the
interior before passing into prairies and plains. On the other hand, the
influence of the Rocky mountains and the Sierra Nevada is very marked.
The latter rise to a great height relatively near the coast, and condense upon
their western slopes nearly all of the moisture brought from the Pacific. The
Rocky mountains have the same effect upon the much drier winds that blow
from the east, and the two systems in consequence enclose a parched desert.
This series of major zones thus becomes, starting at the east, forest,
grassland, desert, and forest, instead of the more symmetrical series, forest,
grassland, desert, grassland, forest, which would prevail were it not for
these barriers. This actual series of major zones undergoes further
interruption by the action of these mountain systems in deflecting northern
isotherms far to the south. This action is greatest in the high ranges, the
Rocky mountains and the Sierras, and least in the lower Appalachians. Its
result is to carry the polar deserts of the north far southward along the crests
of the mountains, and to extend the boreal coniferous forests much further
south along their slopes. In the Appalachians, this means no more than the
extension of a long tongue of conifers into the mass of deciduous forests,
and the occasional appearance of an isolated peak. In the western ranges, it
produces two symmetrical series of minor mountain zones, forest, alpine
grassland or desert, and forest, to say nothing of the foot-hill and timber-
line zones of thicket.
There seems to be no good reason for distinguishing the zones of
mountains as regions. The term itself is inapplicable, as it has no reference
to zonation, and is used much more frequently as a term of general
Page 365
application. Its use tends to obscure also the essential identity of the so-
called vertical zones of mountains with the major continental zones, an
identity which can not be insisted upon too strongly. For the sake of
clearness, it is important to distinguish all belts of vegetation as zones,
though it is evident that these are not all of the same rank. The following
division of the vegetation of North America is based upon the fundamental
principles of continental symmetry and the community of continental and
mountain zones.
I. Polar-niveal zone—zona polari-nivalis
II. Arctic-alpine zone—zona arctici-alpina
Arctic province—provincia arctica
Alpine province—provincia alpina
III. Boreal-subalpine zone—zona boreali-subalpina
Alaska province—provincia alaskana
Cordilleran province—provincia cordillerana
Ontario province—provincia ontariensis
IV. Temperate zone—zona temperata
Atlantic province—provincia atlantica
Appalachian province—provincia appalachiana
Nebraska province—provincia nebraskensis
Utah province—provincia utahensis
Coast province—provincia litoralis
Pacific province—provincia pacifica
V. Subtropical zone—zona subtropicalis
Florida province—provincia floridana
Mexican province—provincia mexicana
called vertical zones of mountains with the major continental zones, an
identity which can not be insisted upon too strongly. For the sake of
clearness, it is important to distinguish all belts of vegetation as zones,
though it is evident that these are not all of the same rank. The following
division of the vegetation of North America is based upon the fundamental
principles of continental symmetry and the community of continental and
mountain zones.
I. Polar-niveal zone—zona polari-nivalis
II. Arctic-alpine zone—zona arctici-alpina
Arctic province—provincia arctica
Alpine province—provincia alpina
III. Boreal-subalpine zone—zona boreali-subalpina
Alaska province—provincia alaskana
Cordilleran province—provincia cordillerana
Ontario province—provincia ontariensis
IV. Temperate zone—zona temperata
Atlantic province—provincia atlantica
Appalachian province—provincia appalachiana
Nebraska province—provincia nebraskensis
Utah province—provincia utahensis
Coast province—provincia litoralis
Pacific province—provincia pacifica
V. Subtropical zone—zona subtropicalis
Florida province—provincia floridana
Mexican province—provincia mexicana
Page 366
VI. Tropical zone—zona tropicalis
Antilles province—provincia antilleana
Andean province—provincia andeana
Antilles province—provincia antilleana
Andean province—provincia andeana
Page 367
ALTERNATION
342. Concept. The term alternation is used to designate that phenomenon
of vegetation, in which a formation recurs at different places in a region, or
a species at separate points in a formation. Although it is a fundamental
feature of vegetation, it has been recognized but recently.[41]
Alternation is the response of vegetation to the heterogeneity of the
surface of the earth. It is in sharp contrast to zonation, inasmuch as it is
directly caused by asymmetry in the topography. In consequence, it deals
with the subdivisions of zones, arising from physical differences within the
symmetrical area. It deals with vegetation areas of every rank below that of
major zone, with the habitat and geographical areas of species, and, in a
certain way, with the correspondence of vicarious genera. The breaking up
of vegetation into formations is a striking example of alternation. The same
phenomenon occurs in every formation, producing consocies and minor
plant groups, and everywhere giving variation to its surface and structure.
The essential idea involved in this principle is the recurrence of like
formations, consocies, or groups, which are more or less separated by
formations, consocies, or groups differing from them. It is an exact
expression of the primary law of association that heterogeneity of structure
varies directly as the extent and complexity of the habitat, or the series of
habitats. Vegetation is made up of what are superficially homogeneous
formations, but upon analysis these are seen to contain consocies. The
latter, though more uniform than formations, break up into groups, each of
which still shows a characteristic heterogeneity arising from the varying
number and arrangement of its constituent species.
343. Causes. The primary cause of alternation is physical asymmetry,
which is everywhere present within the symmetrical areas which produce
zones. This is influenced so strongly, however, by migration and plant
competition (phyteris) that the consideration of this subject will gain in
clearness if these are treated as separate causes. The essential relation
between them must not be lost sight of, however. Migration carries
disseminules into all, or only some of the different areas of a formation, or
into different formations, with little respect to the physical nature of these.
The physical character of these asymmetrical areas determines that some of
342. Concept. The term alternation is used to designate that phenomenon
of vegetation, in which a formation recurs at different places in a region, or
a species at separate points in a formation. Although it is a fundamental
feature of vegetation, it has been recognized but recently.[41]
Alternation is the response of vegetation to the heterogeneity of the
surface of the earth. It is in sharp contrast to zonation, inasmuch as it is
directly caused by asymmetry in the topography. In consequence, it deals
with the subdivisions of zones, arising from physical differences within the
symmetrical area. It deals with vegetation areas of every rank below that of
major zone, with the habitat and geographical areas of species, and, in a
certain way, with the correspondence of vicarious genera. The breaking up
of vegetation into formations is a striking example of alternation. The same
phenomenon occurs in every formation, producing consocies and minor
plant groups, and everywhere giving variation to its surface and structure.
The essential idea involved in this principle is the recurrence of like
formations, consocies, or groups, which are more or less separated by
formations, consocies, or groups differing from them. It is an exact
expression of the primary law of association that heterogeneity of structure
varies directly as the extent and complexity of the habitat, or the series of
habitats. Vegetation is made up of what are superficially homogeneous
formations, but upon analysis these are seen to contain consocies. The
latter, though more uniform than formations, break up into groups, each of
which still shows a characteristic heterogeneity arising from the varying
number and arrangement of its constituent species.
343. Causes. The primary cause of alternation is physical asymmetry,
which is everywhere present within the symmetrical areas which produce
zones. This is influenced so strongly, however, by migration and plant
competition (phyteris) that the consideration of this subject will gain in
clearness if these are treated as separate causes. The essential relation
between them must not be lost sight of, however. Migration carries
disseminules into all, or only some of the different areas of a formation, or
into different formations, with little respect to the physical nature of these.
The physical character of these asymmetrical areas determines that some of
Page 368
these plants shall be established in one series of places, and some in
another, while the competition between the individuals in the various areas
determines the numerical value of each species as well as its persistence.
These three causes are invariably present in the production of alternating
areas, and originally, i. e., in new or denuded soils, the sequence is constant,
viz., migration, ecesis in asymmetrical areas, and competition.
With respect to the different portions of an asymmetrical area, migration
will have one of three effects: (1) it will carry disseminules into both
favorable and unfavorable areas, (2) into favorable ones only, or (3) into
unfavorable ones alone. From the radial nature of migration, the first case is
far the most frequent; it is typical of sporostrotes, and the highly specialized
spermatostrotes and carpostrotes. The effect of migration is uniform here,
and alternation arises in consequence of the selective power of ecesis. It is
evident that migration does not have an even indirect effect, when the
disseminules are carried into none but unfavorable situations. Where the
movement is into favorable places alone, alternation is the immediate result.
The intermittent operation of migration and the presence of barriers are
responsible for the absence of plants in situations favorable to them, and in
consequence bring about a certain alternation between corresponding
species.
The selective operation of physical factors upon the disseminules carried
into the different parts of an asymmetrical area is the usual cause of
alternation. Asymmetry alone is universal within the more conspicuous
structures termed zones, down to the smallest areas which a group of plants
can occupy. The difference between contiguous areas, particularly within
the same habitat, is often small. It sometimes seems inefficient in the initial
stages of a succession when a single species is present, but even in extreme
cases its effect will be recognizable in the size and density of the
individuals. Asymmetry is clearly evident in vegetation where two
symmetrical series cross each other, or when a symmetry is interrupted by
barrier-like elevations or depressions. Within formations, it arises from
differences, often very slight, in slope, exposure, elevation, from
irregularities of surface, differences in soil structure, or composition, in the
amount of cover, and in the reactions of the living plants. At the last point, it
is in direct connection with plant competition.
another, while the competition between the individuals in the various areas
determines the numerical value of each species as well as its persistence.
These three causes are invariably present in the production of alternating
areas, and originally, i. e., in new or denuded soils, the sequence is constant,
viz., migration, ecesis in asymmetrical areas, and competition.
With respect to the different portions of an asymmetrical area, migration
will have one of three effects: (1) it will carry disseminules into both
favorable and unfavorable areas, (2) into favorable ones only, or (3) into
unfavorable ones alone. From the radial nature of migration, the first case is
far the most frequent; it is typical of sporostrotes, and the highly specialized
spermatostrotes and carpostrotes. The effect of migration is uniform here,
and alternation arises in consequence of the selective power of ecesis. It is
evident that migration does not have an even indirect effect, when the
disseminules are carried into none but unfavorable situations. Where the
movement is into favorable places alone, alternation is the immediate result.
The intermittent operation of migration and the presence of barriers are
responsible for the absence of plants in situations favorable to them, and in
consequence bring about a certain alternation between corresponding
species.
The selective operation of physical factors upon the disseminules carried
into the different parts of an asymmetrical area is the usual cause of
alternation. Asymmetry alone is universal within the more conspicuous
structures termed zones, down to the smallest areas which a group of plants
can occupy. The difference between contiguous areas, particularly within
the same habitat, is often small. It sometimes seems inefficient in the initial
stages of a succession when a single species is present, but even in extreme
cases its effect will be recognizable in the size and density of the
individuals. Asymmetry is clearly evident in vegetation where two
symmetrical series cross each other, or when a symmetry is interrupted by
barrier-like elevations or depressions. Within formations, it arises from
differences, often very slight, in slope, exposure, elevation, from
irregularities of surface, differences in soil structure, or composition, in the
amount of cover, and in the reactions of the living plants. At the last point, it
is in direct connection with plant competition.
Page 369
344. Competition. Much uncertainty, as well as diversity of opinion,
seems still to exist in regard to the precise nature of the competition
between plants that occupy the same area. It has long been admitted that the
phrase, “struggle for existence,” is true of this relation only in the most
figurative sense, but the feeling still prevails that, since plants live in
associations, there must be something mysterious and vitalistic in their
relation. No one has been able to discover anything of this nature, but
nevertheless the impression remains. Such a direct relation exists only
between parasites, epiphytes, and lianes, and the plants which serve to
nourish or support them. In the case of plants growing on the same stratum,
actual competition between plant and plant does not occur. One individual
can affect another only in as much as it changes the physical factors that
influence the latter. Competition is a question of the reaction of a plant upon
the physical factors which encompass it, and of the effect of these modified
factors upon the adjacent plants. In the exact sense, two plants do not
compete with each other as long as the water-content and nutrition, the heat
and light are in excess of the needs of both. The moment, however, that the
roots of one enter the area from which the other draws its water supply, or
the foliage of one begins to overshade the leaves of the other, the reaction of
the former modifies unfavorably the factors controlling the latter, and
competition is at once initiated. The same relation exists throughout the
process; the stronger, taller, the more branched, or the better rooted plant
reacts upon the habitat, and the latter immediately exerts an unfavorable
effect upon the weaker, shorter, less branched, or more poorly rooted plant.
This action of plant upon habitat and of habitat upon plant is cumulative,
however. An increase in the leaf surface of a plant not merely reduces the
amount of light and heat available for the plant near it or beneath it, but it
also renders necessary the absorption of more water and other nutritive
material, and correspondingly decreases the amount available. The
inevitable result is that the successful individual prospers more and more,
while the less successful one loses ground in the same degree. As a
consequence, the latter disappears entirely, or it is handicapped to such an
extent that it fails to produce seeds, or these are reduced in number or
vitality.
Competition in vegetation furnishes few instances as simple as the above,
but this will serve to make clear the simplest case of ordinary competition,
i. e., that in which the individuals belong to a single species. The various
seems still to exist in regard to the precise nature of the competition
between plants that occupy the same area. It has long been admitted that the
phrase, “struggle for existence,” is true of this relation only in the most
figurative sense, but the feeling still prevails that, since plants live in
associations, there must be something mysterious and vitalistic in their
relation. No one has been able to discover anything of this nature, but
nevertheless the impression remains. Such a direct relation exists only
between parasites, epiphytes, and lianes, and the plants which serve to
nourish or support them. In the case of plants growing on the same stratum,
actual competition between plant and plant does not occur. One individual
can affect another only in as much as it changes the physical factors that
influence the latter. Competition is a question of the reaction of a plant upon
the physical factors which encompass it, and of the effect of these modified
factors upon the adjacent plants. In the exact sense, two plants do not
compete with each other as long as the water-content and nutrition, the heat
and light are in excess of the needs of both. The moment, however, that the
roots of one enter the area from which the other draws its water supply, or
the foliage of one begins to overshade the leaves of the other, the reaction of
the former modifies unfavorably the factors controlling the latter, and
competition is at once initiated. The same relation exists throughout the
process; the stronger, taller, the more branched, or the better rooted plant
reacts upon the habitat, and the latter immediately exerts an unfavorable
effect upon the weaker, shorter, less branched, or more poorly rooted plant.
This action of plant upon habitat and of habitat upon plant is cumulative,
however. An increase in the leaf surface of a plant not merely reduces the
amount of light and heat available for the plant near it or beneath it, but it
also renders necessary the absorption of more water and other nutritive
material, and correspondingly decreases the amount available. The
inevitable result is that the successful individual prospers more and more,
while the less successful one loses ground in the same degree. As a
consequence, the latter disappears entirely, or it is handicapped to such an
extent that it fails to produce seeds, or these are reduced in number or
vitality.
Competition in vegetation furnishes few instances as simple as the above,
but this will serve to make clear the simplest case of ordinary competition,
i. e., that in which the individuals belong to a single species. The various
Page 370
individuals of one species which grow together in a patch show relatively
slight differences, in height, width, leaf expanse, or root surface. Still, some
will have the largest surfaces for the impact of water, heat, and light, while
others will have the smallest; the majority, perhaps, will occupy different
places between the extremes. The former will receive more than their share
of one or more factors. The reaction thus produced will operate upon the
plants subject to it inversely as the amount of surface impinged upon. The
usual expression of such competition is seen in the great variation in height,
branching, etc., of the different individuals, and in the inability of many to
produce flowers. This is particularly true of annuals, and of perennials of
the same generation. In the competition between parents and offspring of
the same perennial species, the former usually have so much the advantage
that the younger plants are often unable to thrive or even germinate, and
disappear, leaving a free space beneath and about the stronger parents. This
illustrates the primary law of competition, viz., that this is closest when the
individuals are most similar. Similar individuals make nearly the same
demands upon the habitat, and adjust themselves least readily to their
mutual reactions. The more unlike plants are, the greater the difference in
their needs, and some are able to adjust themselves to the reactions of
others with little or no disadvantage.
In accordance with the above principles, the competition is closer
between species of like form than between those that are dissimilar. This
similarity must be one of vegetation or habitat form, not one of systematic
position. The latter is in fact of no significance, except where there is a
certain correspondence between the two. Leaf, stem, and root characters
determine the outcome, and those species most alike in these features will
be in close competition, regardless of their taxonomic similarity or
dissimilarity. This is as conclusive of the competition between the species
of the same genus as it is between those belonging to genera of widely
separated families. From this may be deduced a second principle of
competition, viz., the closeness of the competition between the individuals
of different species varies directly with their similarity in vegetation or
habitat form. This principle is of primary importance in the competition
which arises between occupants and invaders in the different stages of
succession. Those invading species that show the greatest resemblance to
occupants in leaf, stem, and root form experience the greatest difficulty in
establishing themselves. The species, on the contrary, which are so unlike
slight differences, in height, width, leaf expanse, or root surface. Still, some
will have the largest surfaces for the impact of water, heat, and light, while
others will have the smallest; the majority, perhaps, will occupy different
places between the extremes. The former will receive more than their share
of one or more factors. The reaction thus produced will operate upon the
plants subject to it inversely as the amount of surface impinged upon. The
usual expression of such competition is seen in the great variation in height,
branching, etc., of the different individuals, and in the inability of many to
produce flowers. This is particularly true of annuals, and of perennials of
the same generation. In the competition between parents and offspring of
the same perennial species, the former usually have so much the advantage
that the younger plants are often unable to thrive or even germinate, and
disappear, leaving a free space beneath and about the stronger parents. This
illustrates the primary law of competition, viz., that this is closest when the
individuals are most similar. Similar individuals make nearly the same
demands upon the habitat, and adjust themselves least readily to their
mutual reactions. The more unlike plants are, the greater the difference in
their needs, and some are able to adjust themselves to the reactions of
others with little or no disadvantage.
In accordance with the above principles, the competition is closer
between species of like form than between those that are dissimilar. This
similarity must be one of vegetation or habitat form, not one of systematic
position. The latter is in fact of no significance, except where there is a
certain correspondence between the two. Leaf, stem, and root characters
determine the outcome, and those species most alike in these features will
be in close competition, regardless of their taxonomic similarity or
dissimilarity. This is as conclusive of the competition between the species
of the same genus as it is between those belonging to genera of widely
separated families. From this may be deduced a second principle of
competition, viz., the closeness of the competition between the individuals
of different species varies directly with their similarity in vegetation or
habitat form. This principle is of primary importance in the competition
which arises between occupants and invaders in the different stages of
succession. Those invading species that show the greatest resemblance to
occupants in leaf, stem, and root form experience the greatest difficulty in
establishing themselves. The species, on the contrary, which are so unlike
Page 371
the occupants that they come in at a clear advantage or disadvantage,
establish themselves readily, in the one case as a result of the reaction, in
the other by taking a subordinate position. This principle lies at the base of
the changes in succession which give a peculiar stamp to each stage. A
reaction sufficient to bring about the disappearance of one stage can be
produced only by the entrance of invaders so different in form as to
materially or entirely change the impress of the formation. Stabilization
results when the entrance of invaders of such form as to exert an efficient
reaction is no longer possible. In forests, while many vegetation forms can
still enter, none of these produce a reaction sufficient to place the trees at a
disadvantage, and the ultimate forest stage, though it may change in
composition, can not be displaced by another.
It is obvious that the vegetation forms and habitat forms of associated
species are of fundamental importance in determining the course and result
of competition. Identity of vegetation form regularly produces close
competition, and the consequent numerical reduction or disappearance of
one or more species. Dissimilarity, on the other hand, tends to eliminate
competition, and to preserve the advantage of the superior form. Species of
trees compete sharply with each other when found together; the same is true
of shrubs, or rosettes, etc. The relation of the shrubs to the trees, or of the
rosettes to the shrubs of a formation is one of subordination rather than of
competition. The matter of height and width often enters here also to such a
degree that the tallest herbs compete with the bushes and shrubs, and
rosettes with mats or grasses. The amount and disposition of the leaf surface
are decisive factors in the competition between species of the same
vegetation form, in so far as this is governed by light. In those plants in
which the leaves are usually erect, notably the grasses and sedges, the
competition between the aerial parts is relatively slight, and the result is
determined by the reactions of the underground stems and roots.
The position of the competing individuals is of the greatest importance.
The distance between the plants affects directly the degree of competition,
while their arrangement, whether in groups according to species or singly,
exerts a marked influence by determining that the contest shall be between
like forms, or unlike forms. Position is controlled primarily by the relation
existing between seed-production and dissemination. It is of course
influenced in large measure by the initial position taken by the invaders into
a nudate area, but this is itself a result of the same phenomena. The
establish themselves readily, in the one case as a result of the reaction, in
the other by taking a subordinate position. This principle lies at the base of
the changes in succession which give a peculiar stamp to each stage. A
reaction sufficient to bring about the disappearance of one stage can be
produced only by the entrance of invaders so different in form as to
materially or entirely change the impress of the formation. Stabilization
results when the entrance of invaders of such form as to exert an efficient
reaction is no longer possible. In forests, while many vegetation forms can
still enter, none of these produce a reaction sufficient to place the trees at a
disadvantage, and the ultimate forest stage, though it may change in
composition, can not be displaced by another.
It is obvious that the vegetation forms and habitat forms of associated
species are of fundamental importance in determining the course and result
of competition. Identity of vegetation form regularly produces close
competition, and the consequent numerical reduction or disappearance of
one or more species. Dissimilarity, on the other hand, tends to eliminate
competition, and to preserve the advantage of the superior form. Species of
trees compete sharply with each other when found together; the same is true
of shrubs, or rosettes, etc. The relation of the shrubs to the trees, or of the
rosettes to the shrubs of a formation is one of subordination rather than of
competition. The matter of height and width often enters here also to such a
degree that the tallest herbs compete with the bushes and shrubs, and
rosettes with mats or grasses. The amount and disposition of the leaf surface
are decisive factors in the competition between species of the same
vegetation form, in so far as this is governed by light. In those plants in
which the leaves are usually erect, notably the grasses and sedges, the
competition between the aerial parts is relatively slight, and the result is
determined by the reactions of the underground stems and roots.
The position of the competing individuals is of the greatest importance.
The distance between the plants affects directly the degree of competition,
while their arrangement, whether in groups according to species or singly,
exerts a marked influence by determining that the contest shall be between
like forms, or unlike forms. Position is controlled primarily by the relation
existing between seed-production and dissemination. It is of course
influenced in large measure by the initial position taken by the invaders into
a nudate area, but this is itself a result of the same phenomena. The
Page 372
individuals of species with great seed-production and little or no mobility
usually occur in dense stands. In these, the competition is fierce, for the two
reasons of similarity and density, and the result is that the plants fall far
below the normal in height and width. This is an extreme example of the
group arrangement. When the seed-production is small, the mobility may be
great or little without seriously affecting the result. The individuals of a
species of this kind will be scattered among those of other species, and the
closeness of competition will depend largely upon the similarity existing
between the two. The arrangement in such cases is sparse. A species with
great seed-production and great mobility usually shows both kinds of
arrangement, the position of the individuals and the competition between
them varying accordingly. This is due to the intermittent action of
distributing agents, making it possible for the seeds to fall directly to the
ground during the times that winds, etc., are absent. The three types of
arrangement indicated above are termed gregarious, copious, and gregario-
copious. They furnish the basis for the investigation of abundance which
deals essentially with the number and arrangement of the individuals of
competing species. The effect of distance, i. e., the interval between
individuals, upon competition is fundamental. The competition increases as
the interval diminishes, and the reverse.
The view here advanced, i. e., that competition is purely physical in
nature, renders untenable the current conceptions of vegetation pressure,
occupation, etc. Masses of vegetation are thought to force the weaker
species toward the edge, thus initiating an outward or forward pressure. As
has been shown above, no such phenomenon occurs in vegetation. This
movement is nothing but simple migration, followed by ecesis, and has no
connection with “weaker” species, or the development of a vital pressure.
The direction taken by the migrating disseminules is essentially
indeterminate. Migration seems to be outward, or away from the mass,
merely because the ecesis is greater at the edge, where the increased
dissimilarity between plant forms diminishes the competition. The actual
movement is outward, but it takes place through the normal operation of
competition. In this connection, it should be pointed out that the common
view that plants require room is inexact, if not erroneous. This is difficult of
proof, as it is impossible to distinguish room as such from the factors
normally present, light, heat, water, and nutrient salts, but it seems obvious
that the available amounts of these will determine the space occupied by a
usually occur in dense stands. In these, the competition is fierce, for the two
reasons of similarity and density, and the result is that the plants fall far
below the normal in height and width. This is an extreme example of the
group arrangement. When the seed-production is small, the mobility may be
great or little without seriously affecting the result. The individuals of a
species of this kind will be scattered among those of other species, and the
closeness of competition will depend largely upon the similarity existing
between the two. The arrangement in such cases is sparse. A species with
great seed-production and great mobility usually shows both kinds of
arrangement, the position of the individuals and the competition between
them varying accordingly. This is due to the intermittent action of
distributing agents, making it possible for the seeds to fall directly to the
ground during the times that winds, etc., are absent. The three types of
arrangement indicated above are termed gregarious, copious, and gregario-
copious. They furnish the basis for the investigation of abundance which
deals essentially with the number and arrangement of the individuals of
competing species. The effect of distance, i. e., the interval between
individuals, upon competition is fundamental. The competition increases as
the interval diminishes, and the reverse.
The view here advanced, i. e., that competition is purely physical in
nature, renders untenable the current conceptions of vegetation pressure,
occupation, etc. Masses of vegetation are thought to force the weaker
species toward the edge, thus initiating an outward or forward pressure. As
has been shown above, no such phenomenon occurs in vegetation. This
movement is nothing but simple migration, followed by ecesis, and has no
connection with “weaker” species, or the development of a vital pressure.
The direction taken by the migrating disseminules is essentially
indeterminate. Migration seems to be outward, or away from the mass,
merely because the ecesis is greater at the edge, where the increased
dissimilarity between plant forms diminishes the competition. The actual
movement is outward, but it takes place through the normal operation of
competition. In this connection, it should be pointed out that the common
view that plants require room is inexact, if not erroneous. This is difficult of
proof, as it is impossible to distinguish room as such from the factors
normally present, light, heat, water, and nutrient salts, but it seems obvious
that the available amounts of these will determine the space occupied by a
Page 373
plant, irrespective of the room adjacent plants may allow it. The explanation
of competition upon physical grounds likewise invalidates the view that
plants possess spheres of influence other than the areas within which they
exert a demonstrable reaction upon the physical factors present.
Competition plays a very important role in alternation. It produces minor
examples of alternation in the physical units of an asymmetrical series. Its
greatest influence, however, is exerted in modifying the effects of
asymmetry. The reaction of occupants emphasizes or reduces the effect of
asymmetry, and has a corresponding action upon alternation. This result of
competition is typical of succession, in which the sequence of stages arises
from the interaction of occupant and invader.
345. Kinds of alternation. Alternation involves two ideas, viz., the
alternation of different species or formations with each other, and the
alternation of the same species or formation in similar but separate
situations. This is the evident result of asymmetry, in response to which
contiguous areas are dissimilar and remote ones often similar. Individuals of
the same species or examples of the same formation may be said to
alternate between two or more similar situations, while different species or
formations are said to alternate with each other, occurring usually in
situations different in character. From the nature of alternation, the two
phenomena are invariably found together.
It is possible to distinguish three kinds of alternation: (1) of a formation,
consocies, layer, facies, or species in similar situations; (2) of similar or
corresponding formations, species, etc., in similar situations; (3) of facies
and other species with respect to number. The last two are merely variations
of the first, arising out of slight differences in the physical factors of the
alternating areas, the adjacent flora, or the course of competition. The
alternation of different examples of the same formation is a significant
feature of greatly diversified areas, such as mountains. It is naturally much
less characteristic of lands physiographically more uniform. A xerophytic
formation will alternate from ridge to ridge, a mesophytic formation
between the intermediate valleys; aquatic vegetation will alternate from
pond to pond, or stream to stream. The appearance of new or denuded soils
upon which successions establish themselves is the most important cause of
the alternation of formations. The weathering of rocks in different areas of
the same region produces in each a sequence of similar or identical
of competition upon physical grounds likewise invalidates the view that
plants possess spheres of influence other than the areas within which they
exert a demonstrable reaction upon the physical factors present.
Competition plays a very important role in alternation. It produces minor
examples of alternation in the physical units of an asymmetrical series. Its
greatest influence, however, is exerted in modifying the effects of
asymmetry. The reaction of occupants emphasizes or reduces the effect of
asymmetry, and has a corresponding action upon alternation. This result of
competition is typical of succession, in which the sequence of stages arises
from the interaction of occupant and invader.
345. Kinds of alternation. Alternation involves two ideas, viz., the
alternation of different species or formations with each other, and the
alternation of the same species or formation in similar but separate
situations. This is the evident result of asymmetry, in response to which
contiguous areas are dissimilar and remote ones often similar. Individuals of
the same species or examples of the same formation may be said to
alternate between two or more similar situations, while different species or
formations are said to alternate with each other, occurring usually in
situations different in character. From the nature of alternation, the two
phenomena are invariably found together.
It is possible to distinguish three kinds of alternation: (1) of a formation,
consocies, layer, facies, or species in similar situations; (2) of similar or
corresponding formations, species, etc., in similar situations; (3) of facies
and other species with respect to number. The last two are merely variations
of the first, arising out of slight differences in the physical factors of the
alternating areas, the adjacent flora, or the course of competition. The
alternation of different examples of the same formation is a significant
feature of greatly diversified areas, such as mountains. It is naturally much
less characteristic of lands physiographically more uniform. A xerophytic
formation will alternate from ridge to ridge, a mesophytic formation
between the intermediate valleys; aquatic vegetation will alternate from
pond to pond, or stream to stream. The appearance of new or denuded soils
upon which successions establish themselves is the most important cause of
the alternation of formations. The weathering of rocks in different areas of
the same region produces in each a sequence of similar or identical
Page 374
formations. The same statement is true in general of other causes of
succession, such as erosion, flooding, burning, cultivation, etc., wherever
they operate upon areas physically similar and surrounded by the same type
of vegetation. The areas of more or less heterogeneous formations
characterized by major physical differences are occupied by consocies. In
an extensive formation, the same consocies alternates from one to another
of these areas that are similar. When the formation is interrupted and occurs
here and there in separate examples, a consocies often alternates from one
to another of these. A consocies regularly derives its character from the fact
that one or more of the facies of the formation is more intimately connected
with certain areas of the latter than with others. This explains why the
alternations of consocies and facies are usually identical. Layers sometimes
alternate between different examples of the same forest or thicket
formation, when they are suppressed in some by the diffuseness of the light.
The alternation of species is a typical feature of formations; it is absent
only in those rare cases where the latter consist of a single species. The
areas of a habitat which show minor physical or historical (i. e.,
competitive) differences are occupied by groups of individuals belonging to
one or more species responsive to these differences. Each of these groups
will recur in all areas essentially similar, the intervals being occupied of
course by slightly different groups. Such groups are constituted by
gregarious or copious species of restricted adjustability. Sparse plants
likewise alternate, but they necessarily play a much less conspicuous part.
In habitats not too heterogeneous, a large number of species are sufficiently
adjustable to the slight differences so that they occur throughout the
formation. Often, to be sure, they show a characteristic response, expressed
in the size or number. This is illustrated by the facies and many of the
principal species of the prairie formation. Festuca, Koelera, Panicum, and
Andropogon occur throughout, except in the moist ravines which are
practically meadows. Astragalus, Psoralea, Erigeron, and Aster grow
everywhere on slopes and crests, but they are much more abundant in
certain situations. Other plants, Lomatium, Meriolix, Anemone, Pentstemon,
etc., recur in similar or identical situations upon different hills. Lomatium
alternates between sandy or sandstone crests, Meriolix and Pentstemon
occur together upon dry upper slopes, while Anemone alternates between
dry slopes and crests.
succession, such as erosion, flooding, burning, cultivation, etc., wherever
they operate upon areas physically similar and surrounded by the same type
of vegetation. The areas of more or less heterogeneous formations
characterized by major physical differences are occupied by consocies. In
an extensive formation, the same consocies alternates from one to another
of these areas that are similar. When the formation is interrupted and occurs
here and there in separate examples, a consocies often alternates from one
to another of these. A consocies regularly derives its character from the fact
that one or more of the facies of the formation is more intimately connected
with certain areas of the latter than with others. This explains why the
alternations of consocies and facies are usually identical. Layers sometimes
alternate between different examples of the same forest or thicket
formation, when they are suppressed in some by the diffuseness of the light.
The alternation of species is a typical feature of formations; it is absent
only in those rare cases where the latter consist of a single species. The
areas of a habitat which show minor physical or historical (i. e.,
competitive) differences are occupied by groups of individuals belonging to
one or more species responsive to these differences. Each of these groups
will recur in all areas essentially similar, the intervals being occupied of
course by slightly different groups. Such groups are constituted by
gregarious or copious species of restricted adjustability. Sparse plants
likewise alternate, but they necessarily play a much less conspicuous part.
In habitats not too heterogeneous, a large number of species are sufficiently
adjustable to the slight differences so that they occur throughout the
formation. Often, to be sure, they show a characteristic response, expressed
in the size or number. This is illustrated by the facies and many of the
principal species of the prairie formation. Festuca, Koelera, Panicum, and
Andropogon occur throughout, except in the moist ravines which are
practically meadows. Astragalus, Psoralea, Erigeron, and Aster grow
everywhere on slopes and crests, but they are much more abundant in
certain situations. Other plants, Lomatium, Meriolix, Anemone, Pentstemon,
etc., recur in similar or identical situations upon different hills. Lomatium
alternates between sandy or sandstone crests, Meriolix and Pentstemon
occur together upon dry upper slopes, while Anemone alternates between
dry slopes and crests.
Page 375
Owing to the accidents of migration and competition, similar areas within
a habitat are not occupied by the same species, or group of species. A
species found in one area will be replaced in another by a different one of
the same or a different genus. The controlling factors of the area render
imperative an essential identity of vegetation and habitat form, though in
systematic position the plants may be very diverse. Such genera and species
may be termed corresponding. The relation between such plants is
essentially alternation; it should, perhaps, be distinguished from alternation
proper as corresponsive. The prairie formation furnishes a good example of
this on exposed sandy crests, upon which Lomatium, Comandra, and
Pentstemon alternate. Formations exhibit a similar correspondence.
Fig. 74. Numerical alternation of Pinus and Pseudotsuga upon east and west slopes.
a habitat are not occupied by the same species, or group of species. A
species found in one area will be replaced in another by a different one of
the same or a different genus. The controlling factors of the area render
imperative an essential identity of vegetation and habitat form, though in
systematic position the plants may be very diverse. Such genera and species
may be termed corresponding. The relation between such plants is
essentially alternation; it should, perhaps, be distinguished from alternation
proper as corresponsive. The prairie formation furnishes a good example of
this on exposed sandy crests, upon which Lomatium, Comandra, and
Pentstemon alternate. Formations exhibit a similar correspondence.
Fig. 74. Numerical alternation of Pinus and Pseudotsuga upon east and west slopes.
Page 376
All species that alternate show a variation in abundance from one area to
another. Frequently, the difference is slight, and may be ignored, except in
determining abundance. Very often, however, the variation is so great that a
facies may be reduced, numerically, to the rank of a principal species, or
one of the latter to a secondary species. This phenomenon is distinguished
as numerical alternation. It arises from the fact that the similar areas are
sufficiently different to affect the abundance, without producing complete
suppression. It is probable that this result is due almost entirely to
competition. Astragalus crassicarpus grows on all the slopes of the prairie
formation, but on some it has the abundance of a facies, while on others it is
represented by a few scattered individuals. This difference is much more
striking in separate examples of the same formation, particularly when a
normal facies is reduced to the numerical value of a secondary species. This
is a matter of great importance in the study of formations, for it has
doubtless often resulted in mistaking a consocies for a formation.
Alternation furnishes the logical basis for what may be called
comparative phytogeography. The latter is of much broader scope than the
old subject of geographical distribution, for it treats not only of the
distribution of formations and associations as well as of species, but it also
seeks to explain this by means of principles drawn from the relation
between habitat and vegetation. When the latter come to be fully based
upon physical factor investigations, and upon the effects of migration and
competition as shown in alternation, the comparative study of formations
will represent the highest type of phytogeographical activity.
another. Frequently, the difference is slight, and may be ignored, except in
determining abundance. Very often, however, the variation is so great that a
facies may be reduced, numerically, to the rank of a principal species, or
one of the latter to a secondary species. This phenomenon is distinguished
as numerical alternation. It arises from the fact that the similar areas are
sufficiently different to affect the abundance, without producing complete
suppression. It is probable that this result is due almost entirely to
competition. Astragalus crassicarpus grows on all the slopes of the prairie
formation, but on some it has the abundance of a facies, while on others it is
represented by a few scattered individuals. This difference is much more
striking in separate examples of the same formation, particularly when a
normal facies is reduced to the numerical value of a secondary species. This
is a matter of great importance in the study of formations, for it has
doubtless often resulted in mistaking a consocies for a formation.
Alternation furnishes the logical basis for what may be called
comparative phytogeography. The latter is of much broader scope than the
old subject of geographical distribution, for it treats not only of the
distribution of formations and associations as well as of species, but it also
seeks to explain this by means of principles drawn from the relation
between habitat and vegetation. When the latter come to be fully based
upon physical factor investigations, and upon the effects of migration and
competition as shown in alternation, the comparative study of formations
will represent the highest type of phytogeographical activity.
Page 377
THE FORMATION IN DETAIL
346. The rank of the formation. There have been as many different
opinions in regard to the application of the term formation as there are
concerning the group which is to be called a species. In taxonomy, however,
the concept of the species is purely arbitrary, and agreement can not be
hoped for. In vegetation, on the contrary, the connection between formation
and habitat is so close that any application of the term to a division greater
or smaller than the habitat is both illogical and unfortunate. As effect and
cause, it is inevitable that the unit of the vegetative covering, the formation,
should correspond to the unit of the earth’s surface, the habitat. This places
the formation upon a basis which can be accurately determined. It is
imperative, however, to have a clear understanding of what constitutes the
difference between habitats. A society is in entire correspondence with the
physical factors of its area, and the same is true of the vegetation of a
province. Nevertheless, many societies usually occur in a single habitat, and
a province contains many habitats. The final test of a habitat is an efficient
difference in one or more of the direct factors, water-content, humidity, and
light, by virtue of which the plant covering differs in structure and in
species from the areas contiguous to it. A balsam-spruce forest shows
within itself certain differences of physical factors and of structure. The
water-content will range from 20–25 per cent, and the light from .02–.003.
One portion may consist chiefly of Pseudotsuga mucronata, another of
Picea engelmannii, and a third of Picea parryana, or these species may be
intermingled. If, however, this forest is compared with the gravel slide,
which touches it on one side, and the meadow thicket, which meets it on
another, the physical factors and the species both demonstrate that it is the
forest, and not its parts, which corresponds to a distinct physical entity, the
habitat. This test of a formation is superfluous in a great many cases, where
the physiognomy of the contiguous areas is conclusive evidence of their
difference. It is evident also that remote regions which are floristically
distinct, such as the prairies and the steppes, may possess areas physically
almost identical and yet be covered by different formations. This point is
further discussed under classification.
346. The rank of the formation. There have been as many different
opinions in regard to the application of the term formation as there are
concerning the group which is to be called a species. In taxonomy, however,
the concept of the species is purely arbitrary, and agreement can not be
hoped for. In vegetation, on the contrary, the connection between formation
and habitat is so close that any application of the term to a division greater
or smaller than the habitat is both illogical and unfortunate. As effect and
cause, it is inevitable that the unit of the vegetative covering, the formation,
should correspond to the unit of the earth’s surface, the habitat. This places
the formation upon a basis which can be accurately determined. It is
imperative, however, to have a clear understanding of what constitutes the
difference between habitats. A society is in entire correspondence with the
physical factors of its area, and the same is true of the vegetation of a
province. Nevertheless, many societies usually occur in a single habitat, and
a province contains many habitats. The final test of a habitat is an efficient
difference in one or more of the direct factors, water-content, humidity, and
light, by virtue of which the plant covering differs in structure and in
species from the areas contiguous to it. A balsam-spruce forest shows
within itself certain differences of physical factors and of structure. The
water-content will range from 20–25 per cent, and the light from .02–.003.
One portion may consist chiefly of Pseudotsuga mucronata, another of
Picea engelmannii, and a third of Picea parryana, or these species may be
intermingled. If, however, this forest is compared with the gravel slide,
which touches it on one side, and the meadow thicket, which meets it on
another, the physical factors and the species both demonstrate that it is the
forest, and not its parts, which corresponds to a distinct physical entity, the
habitat. This test of a formation is superfluous in a great many cases, where
the physiognomy of the contiguous areas is conclusive evidence of their
difference. It is evident also that remote regions which are floristically
distinct, such as the prairies and the steppes, may possess areas physically
almost identical and yet be covered by different formations. This point is
further discussed under classification.
Page 378
The existing confusion in the matter of formations is due to two causes.
The first arises from the fact that much ecological work has been hasty.
Little or no attention has been given to development, and in consequence
rudimentary and transitory stages of succession have often been described
as formations. Mixed areas in particular have caused trouble. In the second
place, there has been a marked tendency to minimize the need of
thoroughness and training by calling every slightly different area a
formation. A failure to recognize the primary value of alternation has also
contributed materially to this. Alternating facies, and principal species,
when separated from each other, have often been mistaken for formations.
This is a danger that must be fully appreciated and guarded against. In
practically all regions, the same formation is represented by numerous
scattered areas, all showing greater or less differences arising from
alternation. This is especially true of thickly populated regions where virgin
areas are rare. The fact that twenty-five miles intervene to-day between two
small stretches of primitive prairie is permitted to unduly emphasize their
differences. It requires the study of a number of such examples to
counteract this tendency, and to cause one to see clearly that they must have
been at one time merely so many bits of the prairie formation.
In this connection, the lichen and moss groups which are found on rocks
constitute an interesting problem. It is clear that Peltigera and Cladonia,
which grow on the forest floor, and Evernia, Ramalina, and Physcia, which
are found on the trees, are merely constituent species of the forest
formation. The same is true of Cladonia, Urceolaria, and Parmelia, which
are found among the sedges and grasses of alpine meadows. The physical
conditions are essentially those of the formation, and the lichens themselves
are more or less peculiar to it. This is particularly true of the forest, in
which the two strata, bark and moist shaded soil, are present because of the
trees. In the case of granitic rocks, the circumstances are very different. The
species of lichens found on the rocks are not peculiar to the formation, but
they also occur elsewhere. In the forest, Parmelia, Placodium, Physcia,
Rinodina, Urceolaria, Lecanora, Lecidea, etc., occur on the rocks. In the
alpine meadows, the rock groups are composed of Parmelia, Gyrophora,
Cetraria, Acarospora, Lecanora, Lecidea, Buellia, etc. The stratum itself is
physically very different and constitutes a distinct habitat. These groups are
really small formations, which are quite distinct from the surrounding forest
or meadow. This is proven conclusively in many places in the mountains
The first arises from the fact that much ecological work has been hasty.
Little or no attention has been given to development, and in consequence
rudimentary and transitory stages of succession have often been described
as formations. Mixed areas in particular have caused trouble. In the second
place, there has been a marked tendency to minimize the need of
thoroughness and training by calling every slightly different area a
formation. A failure to recognize the primary value of alternation has also
contributed materially to this. Alternating facies, and principal species,
when separated from each other, have often been mistaken for formations.
This is a danger that must be fully appreciated and guarded against. In
practically all regions, the same formation is represented by numerous
scattered areas, all showing greater or less differences arising from
alternation. This is especially true of thickly populated regions where virgin
areas are rare. The fact that twenty-five miles intervene to-day between two
small stretches of primitive prairie is permitted to unduly emphasize their
differences. It requires the study of a number of such examples to
counteract this tendency, and to cause one to see clearly that they must have
been at one time merely so many bits of the prairie formation.
In this connection, the lichen and moss groups which are found on rocks
constitute an interesting problem. It is clear that Peltigera and Cladonia,
which grow on the forest floor, and Evernia, Ramalina, and Physcia, which
are found on the trees, are merely constituent species of the forest
formation. The same is true of Cladonia, Urceolaria, and Parmelia, which
are found among the sedges and grasses of alpine meadows. The physical
conditions are essentially those of the formation, and the lichens themselves
are more or less peculiar to it. This is particularly true of the forest, in
which the two strata, bark and moist shaded soil, are present because of the
trees. In the case of granitic rocks, the circumstances are very different. The
species of lichens found on the rocks are not peculiar to the formation, but
they also occur elsewhere. In the forest, Parmelia, Placodium, Physcia,
Rinodina, Urceolaria, Lecanora, Lecidea, etc., occur on the rocks. In the
alpine meadows, the rock groups are composed of Parmelia, Gyrophora,
Cetraria, Acarospora, Lecanora, Lecidea, Buellia, etc. The stratum itself is
physically very different and constitutes a distinct habitat. These groups are
really small formations, which are quite distinct from the surrounding forest
or meadow. This is proven conclusively in many places in the mountains
Page 379
where areas of the characteristic lichen formations of cliffs are carried by
the fall of rock fragments into forest and meadow, where they persist
without modification. This also shows clearly that the groups on scattered
rocks in the same area are to be regarded as examples of the same cliff
formation, except where the differences are evidently to be ascribed to
development and not to alternation. Where these rock formations can not be
traced to cliffs or magmata with certainty, they must be considered as
antedating the vegetation in which they occur. Often, indeed, especially in
igneous areas, they are relicts of the initial stage of a primary succession.
Finally, they prove their independence of the forest or meadow formation
by initiating a distinct succession within these. Crustaceous groups or
formations yield to foliose ones, and these in turn give way to formations of
mosses, particularly in the forest where the effect of the diffuse light is felt.
From the above, the following rule of formational limitation is obtained:
any area, which shows an essential difference in physical character,
composition, or development from the surrounding formation is a distinct
formation.
the fall of rock fragments into forest and meadow, where they persist
without modification. This also shows clearly that the groups on scattered
rocks in the same area are to be regarded as examples of the same cliff
formation, except where the differences are evidently to be ascribed to
development and not to alternation. Where these rock formations can not be
traced to cliffs or magmata with certainty, they must be considered as
antedating the vegetation in which they occur. Often, indeed, especially in
igneous areas, they are relicts of the initial stage of a primary succession.
Finally, they prove their independence of the forest or meadow formation
by initiating a distinct succession within these. Crustaceous groups or
formations yield to foliose ones, and these in turn give way to formations of
mosses, particularly in the forest where the effect of the diffuse light is felt.
From the above, the following rule of formational limitation is obtained:
any area, which shows an essential difference in physical character,
composition, or development from the surrounding formation is a distinct
formation.
Page 380
Fig. 75. Relict lichen formation in a spruce forest, invaded by rock mosses.
347. The parts of a formation. All the parts which make up the structure
of a formation are directly referable to zonation and alternation, alone or
together, or to the interaction of the two. The principles which underlie this
have already been discussed under the phenomena concerned. It is
necessary to point out further that the structure may be produced in several
ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as
primary and alternation as secondary, (4) by primary alternation and
secondary zonation, (5) by the interaction of the two, as in layered
formations. Though all these methods occur, the first two are relatively rare,
and the resulting structure comparatively imperfect. The typical structure of
formations can best be made clear by the consideration of a prairie which
belongs to the fourth group, and a forest which represents the last.
347. The parts of a formation. All the parts which make up the structure
of a formation are directly referable to zonation and alternation, alone or
together, or to the interaction of the two. The principles which underlie this
have already been discussed under the phenomena concerned. It is
necessary to point out further that the structure may be produced in several
ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as
primary and alternation as secondary, (4) by primary alternation and
secondary zonation, (5) by the interaction of the two, as in layered
formations. Though all these methods occur, the first two are relatively rare,
and the resulting structure comparatively imperfect. The typical structure of
formations can best be made clear by the consideration of a prairie which
belongs to the fourth group, and a forest which represents the last.
Page 381
Fig. 76. Early (prior) aspect of the alpine meadow formation (Carex-Campanula-coryphium),
characterized by Rydbergia grandiflora.
The major divisions of prairie and forest formations are regularly due to
alternation. There is an inherent tendency to the segregation of facies,
arising out of physical or historical reasons, or from a combination of both.
Not all formations show this, but it is characteristic of the great majority of
them. The primary areas which thus arise have been called associations:
they are naturally subordinate to the formation. To avoid the confusion
which inevitably results from using the word association in two different
senses, it is proposed to term this primary division of the formation, a
consociation, or better, a consocies. This term is applied only to an area
characterized by a facies, or less frequently, by two or more facies
uniformly commingled. The consocies of grassland are determined by
grasses, those of forests by trees, etc. From the different position of the
facies in these two types of vegetation such areas are readily seen at all
times in the forest, but they are often concealed in grassland by the tall-
characterized by Rydbergia grandiflora.
The major divisions of prairie and forest formations are regularly due to
alternation. There is an inherent tendency to the segregation of facies,
arising out of physical or historical reasons, or from a combination of both.
Not all formations show this, but it is characteristic of the great majority of
them. The primary areas which thus arise have been called associations:
they are naturally subordinate to the formation. To avoid the confusion
which inevitably results from using the word association in two different
senses, it is proposed to term this primary division of the formation, a
consociation, or better, a consocies. This term is applied only to an area
characterized by a facies, or less frequently, by two or more facies
uniformly commingled. The consocies of grassland are determined by
grasses, those of forests by trees, etc. From the different position of the
facies in these two types of vegetation such areas are readily seen at all
times in the forest, but they are often concealed in grassland by the tall-
Page 382
growing principal species of the various aspects. When definite consocies
are present, they are often found to mingle where they touch, producing
miniature transition areas, and, very rarely, they sometimes leave gaps in
which no facies appears.
Fig. 77. Late (serotinal) aspect of the alpine meadow, characterized by Campanula petiolata,
Rydbergia in fruit.
The seasonal changes of a formation, which are called aspects, are
indicated by changes in composition or structure, which ordinarily
correspond to the three seasons, spring, summer, and autumn. The latter
affect the facies relatively little, especially those of woody vegetation, but
they influence the principal species profoundly, causing a grouping typical
of each aspect. For these areas controlled by principal species, but changing
from aspect to aspect, the term society is proposed. They are prominent
features of the majority of herbaceous formations, where they are often
more striking than the facies. In forests, they occur in the shrubby and
are present, they are often found to mingle where they touch, producing
miniature transition areas, and, very rarely, they sometimes leave gaps in
which no facies appears.
Fig. 77. Late (serotinal) aspect of the alpine meadow, characterized by Campanula petiolata,
Rydbergia in fruit.
The seasonal changes of a formation, which are called aspects, are
indicated by changes in composition or structure, which ordinarily
correspond to the three seasons, spring, summer, and autumn. The latter
affect the facies relatively little, especially those of woody vegetation, but
they influence the principal species profoundly, causing a grouping typical
of each aspect. For these areas controlled by principal species, but changing
from aspect to aspect, the term society is proposed. They are prominent
features of the majority of herbaceous formations, where they are often
more striking than the facies. In forests, they occur in the shrubby and
Page 383
herbaceous layers, and are consequently much less conspicuous than the
facies. A close inspection of the societies formed by principal species shows
that they are far from uniform. Since they usually fail to exhibit distinct
parts, it becomes necessary to approach the question of their structure from
a new standpoint. Such is afforded by aggregation, which yields the
simplest group in vegetation, i. e., that of parent and offspring. This is so
exactly a family in the ordinary sense that there seems to be ample warrant
for violating a canon of terminology by using the word for this group, in
spite of its very different application in taxonomy. It has already been
shown that aggregation further produces a grouping of families, which may
properly be called a community. As they are used here, family and
community become equally applicable to the association of plants, animals,
or man. Both families and communities occur regularly in each society of
the formation, and they represent its two structures. In some cases, all the
families are grouped in communities, two or more of which then form the
society. Very frequently, however, families occur singly, without reference
to a community, and the two then constitute independent parts of the same
area. This is typically the case wherever gregarious species are present,
since these are merely family groups produced by aggregation.
facies. A close inspection of the societies formed by principal species shows
that they are far from uniform. Since they usually fail to exhibit distinct
parts, it becomes necessary to approach the question of their structure from
a new standpoint. Such is afforded by aggregation, which yields the
simplest group in vegetation, i. e., that of parent and offspring. This is so
exactly a family in the ordinary sense that there seems to be ample warrant
for violating a canon of terminology by using the word for this group, in
spite of its very different application in taxonomy. It has already been
shown that aggregation further produces a grouping of families, which may
properly be called a community. As they are used here, family and
community become equally applicable to the association of plants, animals,
or man. Both families and communities occur regularly in each society of
the formation, and they represent its two structures. In some cases, all the
families are grouped in communities, two or more of which then form the
society. Very frequently, however, families occur singly, without reference
to a community, and the two then constitute independent parts of the same
area. This is typically the case wherever gregarious species are present,
since these are merely family groups produced by aggregation.
Page 384
Fig. 78. Calthetum (Caltha leptosepala), a consocies of the alpine bog formation.
Page 385
Fig. 79. Iridile (Iris missouriensis), a society of the aspen formation.
Objection may be made that this analysis of formational structure has
been carried too far, and that some of the structures recognized are mere
interpretations, and not actual facts. Such a criticism will not come from
one who has got beyond the superficial study of formations, for he will at
once recognize that certain probable features of structure have not been
considered. On the other hand, the ecologist or the botanist who has not
made a careful investigation from the standpoints of development and
structure will naturally refrain from expressing an opinion, until he has
obtained an acquaintance at first hand with the facts. Over-refinement is the
usual penalty of intensive work. The unbiased investigator, however, will
not be misled by the suddenness with which new concepts appear. It seems
plausible that the structure of a formation, if not as definite, is at least
nearly as complex as that of an individual plant. Few botanists will insist
that the refinement of tissues and tissue systems has been carried further
than the differentiation of the plant warrants. Yet, if these had been defined
Objection may be made that this analysis of formational structure has
been carried too far, and that some of the structures recognized are mere
interpretations, and not actual facts. Such a criticism will not come from
one who has got beyond the superficial study of formations, for he will at
once recognize that certain probable features of structure have not been
considered. On the other hand, the ecologist or the botanist who has not
made a careful investigation from the standpoints of development and
structure will naturally refrain from expressing an opinion, until he has
obtained an acquaintance at first hand with the facts. Over-refinement is the
usual penalty of intensive work. The unbiased investigator, however, will
not be misled by the suddenness with which new concepts appear. It seems
plausible that the structure of a formation, if not as definite, is at least
nearly as complex as that of an individual plant. Few botanists will insist
that the refinement of tissues and tissue systems has been carried further
than the differentiation of the plant warrants. Yet, if these had been defined
Page 386
within a period of a few years rather than slowly recognized during more
than a century, they would have been called seriously in question. As a
matter of fact, the consocies, under the term association, and the society,
under various names, have been recognized by ecologists for several years.
They are definite phenomena of alternation which can be found anywhere.
The family and the community, though the latter is less distinct in outline,
are equally valid structures, the proof of which anyone can obtain by
thorough methods of study.
348. Nomenclature of the divisions. The suffix -etum is used to
designate a consocies of a formation, e. g., Picetum, Caricetum, etc. When
two or more species characterize the area, the most important, or more
rarely, the two are used. The termination used to designate a society is -ile,
as Asterile, Sedile, Rosile. The suffix which denotes the community is -are,
and for the family, it is -on, viz., Giliare, Bromare, Bidenton, Helianthon,
etc. Layers are indicated by the affix -anum, as Opulasteranum, Verbesina-
Rudbeckianum, etc. It is evident that these suffixes, like the terms to which
they refer, must be used always for the proper divisions if they are to have
any value at all. There has been a marked tendency, for example, to use -
etum in connection with the names of groups of very different rank. It is
hardly necessary to point out that such a practice does not promote
clearness. The following tabular statement will illustrate the application of
both terms and suffixes:
Picea-Pseudotsuga-hylium formation (-ium) Paronychia-Silene-chalicium
Picetum consocies (-etum) Paronychietum
Opulaster-Ribesanum layer (-anum)
Opulasterile society (-ile) Androsacile
Thalictrare community (-are) Festucare
Pirolon family (-on) Arenarion
349. The investigation of a particular formation. A comprehensive and
thorough study of a formation should be based upon as many examples of it
as are accessible. The example which is at once the most typical and the
most accessible is made the base area. This plan saves time and energy,
reduces the number of instruments that are absolutely necessary, and
establishes a common basis for comparison. The inquiry should be made
along four lines, all fundamental to a proper knowledge of the formation.
than a century, they would have been called seriously in question. As a
matter of fact, the consocies, under the term association, and the society,
under various names, have been recognized by ecologists for several years.
They are definite phenomena of alternation which can be found anywhere.
The family and the community, though the latter is less distinct in outline,
are equally valid structures, the proof of which anyone can obtain by
thorough methods of study.
348. Nomenclature of the divisions. The suffix -etum is used to
designate a consocies of a formation, e. g., Picetum, Caricetum, etc. When
two or more species characterize the area, the most important, or more
rarely, the two are used. The termination used to designate a society is -ile,
as Asterile, Sedile, Rosile. The suffix which denotes the community is -are,
and for the family, it is -on, viz., Giliare, Bromare, Bidenton, Helianthon,
etc. Layers are indicated by the affix -anum, as Opulasteranum, Verbesina-
Rudbeckianum, etc. It is evident that these suffixes, like the terms to which
they refer, must be used always for the proper divisions if they are to have
any value at all. There has been a marked tendency, for example, to use -
etum in connection with the names of groups of very different rank. It is
hardly necessary to point out that such a practice does not promote
clearness. The following tabular statement will illustrate the application of
both terms and suffixes:
Picea-Pseudotsuga-hylium formation (-ium) Paronychia-Silene-chalicium
Picetum consocies (-etum) Paronychietum
Opulaster-Ribesanum layer (-anum)
Opulasterile society (-ile) Androsacile
Thalictrare community (-are) Festucare
Pirolon family (-on) Arenarion
349. The investigation of a particular formation. A comprehensive and
thorough study of a formation should be based upon as many examples of it
as are accessible. The example which is at once the most typical and the
most accessible is made the base area. This plan saves time and energy,
reduces the number of instruments that are absolutely necessary, and
establishes a common basis for comparison. The inquiry should be made
along four lines, all fundamental to a proper knowledge of the formation.
Page 387
These lines are: (1) the determination of the factors of the habitat, (2) a
quadrat and a transect study of the structure of the formation, (3) a similar
investigation of development, (4) a floristic study of the contiguous
formation, with special reference to migration. The sequence indicated has
proven to be the most satisfactory, and is to be regarded as all but absolutely
essential. Naturally, this applies only to the order in which the various lines
are to be taken up, as they are carried on together when the work is fully
under way. Since instrument and quadrat methods have already been given
in detail, it is unnecessary that they be repeated. Similarly, the questions
which pertain to structure and development and to the surrounding
vegetation are considered in detail in the pages which precede.
Fig. 80. Eritrichiare (Eritrichium aretioides), a community of the alpine meadow formation.
quadrat and a transect study of the structure of the formation, (3) a similar
investigation of development, (4) a floristic study of the contiguous
formation, with special reference to migration. The sequence indicated has
proven to be the most satisfactory, and is to be regarded as all but absolutely
essential. Naturally, this applies only to the order in which the various lines
are to be taken up, as they are carried on together when the work is fully
under way. Since instrument and quadrat methods have already been given
in detail, it is unnecessary that they be repeated. Similarly, the questions
which pertain to structure and development and to the surrounding
vegetation are considered in detail in the pages which precede.
Fig. 80. Eritrichiare (Eritrichium aretioides), a community of the alpine meadow formation.
Page 388
CLASSIFICATION AND RELATIONSHIP
350. Bases. Formations may be grouped with reference to habitat or kind,
development or position. Classification upon the basis of habitat places
together formations which are similar in physiognomy and structure.
Developmental classification is based upon the fact that the stages of a
particular succession are organically connected or related, though they are
normally different in both physiognomy and structure. Grouping with
respect to position is made solely upon occurrence in the same division of
vegetation. The formations thus brought together usually possess neither
similarity of kind or structure, nor do they have any necessary
developmental connection. Habitat and developmental classification are of
fundamental value; regional arrangement is more superficial in character.
All serve, however, to emphasize different relations, and, while the
developmental system expresses the most, they should all be used to exhibit
the vegetation of a region, province, or zone.
350. Bases. Formations may be grouped with reference to habitat or kind,
development or position. Classification upon the basis of habitat places
together formations which are similar in physiognomy and structure.
Developmental classification is based upon the fact that the stages of a
particular succession are organically connected or related, though they are
normally different in both physiognomy and structure. Grouping with
respect to position is made solely upon occurrence in the same division of
vegetation. The formations thus brought together usually possess neither
similarity of kind or structure, nor do they have any necessary
developmental connection. Habitat and developmental classification are of
fundamental value; regional arrangement is more superficial in character.
All serve, however, to emphasize different relations, and, while the
developmental system expresses the most, they should all be used to exhibit
the vegetation of a region, province, or zone.
Page 389
Fig. 81. Pachylophon (Pachylophus caespitosus), a family of the gravel slide formation.
351. Habitat classification. In arranging formations with reference to
habitats, the direct factors, water and light, can alone be used to advantage.
Such a system is fundamental, because it is founded upon similarity of
habitat and of structure. Proposed groupings based upon nutrition-content,
or upon the division of factors into climatic and edaphic, have elsewhere[42]
been shown to be altogether of secondary importance, if not actually
erroneous. The basis of the habitat grouping is water-content, which is
supplemented by light whenever the factor is decisive. The primary
divisions thus obtained are water, forest, grassland, and desert, which are
characterized respectively by associations of hydrophytes, mesophytes,
hylophytes, poophytes, and xerophytes respectively. Within these,
formations are arranged according to the type of habitat, i. e., pond,
meadow, forest, dune, etc. These divisions comprise all formations which
belong to the type by virtue of their physiognomy and structure. Such
351. Habitat classification. In arranging formations with reference to
habitats, the direct factors, water and light, can alone be used to advantage.
Such a system is fundamental, because it is founded upon similarity of
habitat and of structure. Proposed groupings based upon nutrition-content,
or upon the division of factors into climatic and edaphic, have elsewhere[42]
been shown to be altogether of secondary importance, if not actually
erroneous. The basis of the habitat grouping is water-content, which is
supplemented by light whenever the factor is decisive. The primary
divisions thus obtained are water, forest, grassland, and desert, which are
characterized respectively by associations of hydrophytes, mesophytes,
hylophytes, poophytes, and xerophytes respectively. Within these,
formations are arranged according to the type of habitat, i. e., pond,
meadow, forest, dune, etc. These divisions comprise all formations which
belong to the type by virtue of their physiognomy and structure. Such
Page 390
formations differ from each other very considerably or completely in the
matter of floristic, i. e., component species, but they still belong to the same
type. A dune formation in the interior and one on the coast may not have a
single species in common, and yet they are essentially alike in habitat,
development, and structure.
352. Nomenclature. The names of formations are taken from the habitats
which they occupy. Each formation should have a vernacular and a
scientific name. The latter is especially important since it ensures brevity
and uniformity, and obviates the obscurity and confusion that arise from
vernacular terms in many tongues. Scientific names have been made
uniformly from Greek words of proper meaning by the addition of the
suffix -ium (εῖον), which denotes place.[43] The following list gives the
English and the scientific name of the various habitats, and their
corresponding formations, and indicates the primary divisions into which
these fall.
I. Hydrophytia: water plant formations
1. ocean: oceanium: oceanad,[44] oceanophilous, etc.
2. sea: thalassium
surface of the sea: pelagium
deep sea: pontium
3. lake: limnium, limnad
4. pond, pool, tiphium, tiphad
5. stagnant water: stasium: stasad
6. salt marsh: limnodium, limnodad
7. fresh marsh: helium
8. wet meadow: telmatium
9. river: potamium
10. creek: rhoium
11. brook: namatium
matter of floristic, i. e., component species, but they still belong to the same
type. A dune formation in the interior and one on the coast may not have a
single species in common, and yet they are essentially alike in habitat,
development, and structure.
352. Nomenclature. The names of formations are taken from the habitats
which they occupy. Each formation should have a vernacular and a
scientific name. The latter is especially important since it ensures brevity
and uniformity, and obviates the obscurity and confusion that arise from
vernacular terms in many tongues. Scientific names have been made
uniformly from Greek words of proper meaning by the addition of the
suffix -ium (εῖον), which denotes place.[43] The following list gives the
English and the scientific name of the various habitats, and their
corresponding formations, and indicates the primary divisions into which
these fall.
I. Hydrophytia: water plant formations
1. ocean: oceanium: oceanad,[44] oceanophilous, etc.
2. sea: thalassium
surface of the sea: pelagium
deep sea: pontium
3. lake: limnium, limnad
4. pond, pool, tiphium, tiphad
5. stagnant water: stasium: stasad
6. salt marsh: limnodium, limnodad
7. fresh marsh: helium
8. wet meadow: telmatium
9. river: potamium
10. creek: rhoium
11. brook: namatium
Page 391
12. torrent: rhyacium
13. spring: crenium
14. warm spring: thermium
15. ditch: taphrium
16. sewer: laurium
17. swamp forest: helohylium
18. swamp open woodland: helodium
19. meadow thicket: helodrium
20. bank: ochthium
rock bank: petrochthium
sand bank: ammochthium
mud bank: pelochthium
21. rocky seashore: actium
22. sandy seashore: agium
23. sandbar: cheradium
24. tank: phretium
II. Mesophytia: middle plant formations
a. Sciophytia: shade plant formations
26. forest: hylium
27. grove: alsium
28. orchard: dendrium
29. canyon: ancium
30. open woodland: orgadium
31. thicket: lochmium
b. Heliophytia: sun plant formations
13. spring: crenium
14. warm spring: thermium
15. ditch: taphrium
16. sewer: laurium
17. swamp forest: helohylium
18. swamp open woodland: helodium
19. meadow thicket: helodrium
20. bank: ochthium
rock bank: petrochthium
sand bank: ammochthium
mud bank: pelochthium
21. rocky seashore: actium
22. sandy seashore: agium
23. sandbar: cheradium
24. tank: phretium
II. Mesophytia: middle plant formations
a. Sciophytia: shade plant formations
26. forest: hylium
27. grove: alsium
28. orchard: dendrium
29. canyon: ancium
30. open woodland: orgadium
31. thicket: lochmium
b. Heliophytia: sun plant formations
Page 392
32. meadow: poium
33. pasture: nomium
34. culture land: agrium
35. waste place: chledium
III. Xerophytia: dry plant formations
36. desert: eremium
37. sand-hills, sandy plain: amathium
38. prairie, plains: psilium
39. dry, open woodland: hylodium
40. dry thicket: driodium
41. dry forest: xerohylium
42. gravel slide: chalicium
43. sandbar: syrtidium
44. sand draw: enaulium
45. blowout: anemium
46. strand: psamathium
47. dune: thinium
48. badlands: tirium
49. hill, ridge: lophium
50. cliff: cremnium
51. rock field: phellium
52. boulder field: petrodium
53. rock, stone: petrium
54. humus marsh: oxodium
55. alkali area: drimium
33. pasture: nomium
34. culture land: agrium
35. waste place: chledium
III. Xerophytia: dry plant formations
36. desert: eremium
37. sand-hills, sandy plain: amathium
38. prairie, plains: psilium
39. dry, open woodland: hylodium
40. dry thicket: driodium
41. dry forest: xerohylium
42. gravel slide: chalicium
43. sandbar: syrtidium
44. sand draw: enaulium
45. blowout: anemium
46. strand: psamathium
47. dune: thinium
48. badlands: tirium
49. hill, ridge: lophium
50. cliff: cremnium
51. rock field: phellium
52. boulder field: petrodium
53. rock, stone: petrium
54. humus marsh: oxodium
55. alkali area: drimium
Page 393
56. heath, dry meadow: xeropoium
57. moor: sterrhium
58. alpine meadow: coryphium
59. polar barrens: crymium
60. snow: chionium
61. wastes: chersium
Particular formations are indicated by means of floristic distinctions.
Thus, Populus-hylium is the aspen forest as distinguished from the Picea-
Pseudotsuga-hylium, or the balsam-spruce forest; and the Bulbilis-psilium,
or buffalo-grass prairie, from the Bouteloua-Andropogon-psilium, or grama-
bluestem prairie. Similarly, the aspen formation of the Old World and of the
New may be distinguished as Populus-tremula-hylium and Populus-
tremuloides-hylium, respectively. In all formational names, the facies alone
should be used. Frequently, a single facies will suffice for clearness. As a
rule, however, the two most important facies should be employed; in rare
cases only is it necessary to use the names of three. When it is desirable to
refer to two or more examples of the same formation, a geographical term is
added, e. g., (1) Populus-hylium (Crystal Park), (2) Populus-hylium (Cabin
Canyon).
353. Developmental classification. This is based upon succession as the
record of development. Upon the basis of development, all the formations
which belong to the same succession are classed together. They are
arranged within each group in the sequence found in the particular
succession. From its nature, developmental classification is of primary
importance in exhibiting the history of vegetational changes. It has less
value than the habitat system for summarizing the essential structure of a
vegetation, inasmuch as it places the emphasis upon historical rather than
structural features. It is evident that both deal with the same formations, and
that the difference is merely one of viewpoint. The habitat classification is
simpler in that it considers only those formations actually on the ground,
while development has regularly to take into account stages which have
disappeared. The groups of the developmental system, and the arrangement
of formations within them have already been indicated under the
nomenclature of succession (sections 326 and 327).
57. moor: sterrhium
58. alpine meadow: coryphium
59. polar barrens: crymium
60. snow: chionium
61. wastes: chersium
Particular formations are indicated by means of floristic distinctions.
Thus, Populus-hylium is the aspen forest as distinguished from the Picea-
Pseudotsuga-hylium, or the balsam-spruce forest; and the Bulbilis-psilium,
or buffalo-grass prairie, from the Bouteloua-Andropogon-psilium, or grama-
bluestem prairie. Similarly, the aspen formation of the Old World and of the
New may be distinguished as Populus-tremula-hylium and Populus-
tremuloides-hylium, respectively. In all formational names, the facies alone
should be used. Frequently, a single facies will suffice for clearness. As a
rule, however, the two most important facies should be employed; in rare
cases only is it necessary to use the names of three. When it is desirable to
refer to two or more examples of the same formation, a geographical term is
added, e. g., (1) Populus-hylium (Crystal Park), (2) Populus-hylium (Cabin
Canyon).
353. Developmental classification. This is based upon succession as the
record of development. Upon the basis of development, all the formations
which belong to the same succession are classed together. They are
arranged within each group in the sequence found in the particular
succession. From its nature, developmental classification is of primary
importance in exhibiting the history of vegetational changes. It has less
value than the habitat system for summarizing the essential structure of a
vegetation, inasmuch as it places the emphasis upon historical rather than
structural features. It is evident that both deal with the same formations, and
that the difference is merely one of viewpoint. The habitat classification is
simpler in that it considers only those formations actually on the ground,
while development has regularly to take into account stages which have
disappeared. The groups of the developmental system, and the arrangement
of formations within them have already been indicated under the
nomenclature of succession (sections 326 and 327).
Page 394
354. Regional classification. The grouping of formations with respect to
the divisions of vegetations is chiefly of geographical value. It indicates a
certain general relationship, but its principal use is to summarize the
structure of the vegetative covering of a region. The arrangement of
formations in the various divisions is made with reference to the outline of
North American vegetation (section 341). This is naturally based upon the
identity of altitude and latitude zones. In the study of mountain countries, it
is often desirable to group formations with reference to altitude alone. In
this case, the grouping is based upon the following divisions: (1)
bathyphytia, lowland plant formations; (2) mesiophytia, midland
formations; (3) pediophytia, upland formations; (4) pagophytia, foot-hill
formations; (5) orophytia, subalpine formations; (6) acrophytia, alpine
formations; (7) chionophytia, niveal formations.
355. Mixed formations. These are mixtures of two, rarely more, adjacent
formations, or of two consecutive stages of the same succession. Mixed
formations are really transitions in space or in time between two distinct
formations. Theoretically, they are to be referred to one or the other,
according to the preponderance of species. Actually, however, they often
persist in an intermediate condition for many years, and it becomes
necessary to devote considerable attention to them. In some cases, there is
good reason to think that the species of two contiguous formations have
become permanently associated, and thus constitute a new formation. This
is often apparently true in succession, when the change from one stage to
the next requires a long term of years, but it is really true only of the very
rare cases in which a succession becomes stabilized in a transition stage.
When the mixture is due to development, the formations concerned are
often quite dissimilar, e. g., grassland and thicket, thicket and forest. If it is
the result of position, the formations are usually similar, i. e., both are
grassland, thicket, or forest, since the plants of the lower level are regularly
assimilated or destroyed, when invasion occurs at two levels. The term
mictium (μικτόν, mixture) is here proposed for the designation of all mixed
formations, whether they arise from succession or from juxtaposition. Thus,
the Mentzelia-Elymus-mictium is the transition between the Mentzelia-
Pseudocymopterus-chalicium and the Elymus-Muhlenbergia-chalicium.
Similarly, the Populus-Picea-mictium and the Pinus-Pseudotsuga-mictium
are transition stages in the development of the Picea-hylium. On the other
hand, the Andropogon-Bulbilis-mictium is a mixture produced by the
the divisions of vegetations is chiefly of geographical value. It indicates a
certain general relationship, but its principal use is to summarize the
structure of the vegetative covering of a region. The arrangement of
formations in the various divisions is made with reference to the outline of
North American vegetation (section 341). This is naturally based upon the
identity of altitude and latitude zones. In the study of mountain countries, it
is often desirable to group formations with reference to altitude alone. In
this case, the grouping is based upon the following divisions: (1)
bathyphytia, lowland plant formations; (2) mesiophytia, midland
formations; (3) pediophytia, upland formations; (4) pagophytia, foot-hill
formations; (5) orophytia, subalpine formations; (6) acrophytia, alpine
formations; (7) chionophytia, niveal formations.
355. Mixed formations. These are mixtures of two, rarely more, adjacent
formations, or of two consecutive stages of the same succession. Mixed
formations are really transitions in space or in time between two distinct
formations. Theoretically, they are to be referred to one or the other,
according to the preponderance of species. Actually, however, they often
persist in an intermediate condition for many years, and it becomes
necessary to devote considerable attention to them. In some cases, there is
good reason to think that the species of two contiguous formations have
become permanently associated, and thus constitute a new formation. This
is often apparently true in succession, when the change from one stage to
the next requires a long term of years, but it is really true only of the very
rare cases in which a succession becomes stabilized in a transition stage.
When the mixture is due to development, the formations concerned are
often quite dissimilar, e. g., grassland and thicket, thicket and forest. If it is
the result of position, the formations are usually similar, i. e., both are
grassland, thicket, or forest, since the plants of the lower level are regularly
assimilated or destroyed, when invasion occurs at two levels. The term
mictium (μικτόν, mixture) is here proposed for the designation of all mixed
formations, whether they arise from succession or from juxtaposition. Thus,
the Mentzelia-Elymus-mictium is the transition between the Mentzelia-
Pseudocymopterus-chalicium and the Elymus-Muhlenbergia-chalicium.
Similarly, the Populus-Picea-mictium and the Pinus-Pseudotsuga-mictium
are transition stages in the development of the Picea-hylium. On the other
hand, the Andropogon-Bulbilis-mictium is a mixture produced by the
Page 395
mingling of two contiguous prairie formations. In the future development of
this subject, it will probably become desirable to name mixed formations on
the basis of origin, but at present this is unnecessary. Both in classification
and in description they should be considered between the formations which
give rise to them, and this will at once indicate their origin.
Fig. 82. A mixed formation of aspens and spruces (Populus-Picea-mictium), preceding the final
spruce forest of a burn succession.
Puzzling cases of mixture resulting from position occur toward the limits
of facies which occupy extensive areas. Bouteloua oligostachya, and
Andropogon scoparius extend from the prairies through the sand-hills and
plains, and into the foot-hills of the Rocky mountains. Their abundance at
once raises a question as to the validity of the prairie, sand-hill, plain, and
foot-hill formations. If these two grasses were controlling, and equally
characteristic throughout, then the entire stretch would have to be regarded
as a single formation. Since they are often absent, or mixed with other
this subject, it will probably become desirable to name mixed formations on
the basis of origin, but at present this is unnecessary. Both in classification
and in description they should be considered between the formations which
give rise to them, and this will at once indicate their origin.
Fig. 82. A mixed formation of aspens and spruces (Populus-Picea-mictium), preceding the final
spruce forest of a burn succession.
Puzzling cases of mixture resulting from position occur toward the limits
of facies which occupy extensive areas. Bouteloua oligostachya, and
Andropogon scoparius extend from the prairies through the sand-hills and
plains, and into the foot-hills of the Rocky mountains. Their abundance at
once raises a question as to the validity of the prairie, sand-hill, plain, and
foot-hill formations. If these two grasses were controlling, and equally
characteristic throughout, then the entire stretch would have to be regarded
as a single formation. Since they are often absent, or mixed with other
Page 396
facies of greater importance, they can not be considered the sole tests of the
formation. This view is reinforced by the fact that prairie, sand-hill, plains,
and foot-hill all have their characteristic principal and secondary species, in
addition to facies that are more or less typical. In certain formations,
doubtless, Bouteloua and Andropogon are relicts, in others invaders, while
in the formations actually constituted by them they are dominant. The final
solution of such problems is quite impossible, however, until the
comparative study of large areas can be based upon the accurate detailed
investigation of the component formations.
formation. This view is reinforced by the fact that prairie, sand-hill, plains,
and foot-hill all have their characteristic principal and secondary species, in
addition to facies that are more or less typical. In certain formations,
doubtless, Bouteloua and Andropogon are relicts, in others invaders, while
in the formations actually constituted by them they are dominant. The final
solution of such problems is quite impossible, however, until the
comparative study of large areas can be based upon the accurate detailed
investigation of the component formations.
Page 397
Experimental Vegetation
356. Scope and methods. The experimental study of the formation as a
complex organism rests upon methods essentially similar to those discussed
under experimental evolution. The scope of the two fields is practically the
same, moreover, in that both deal with the experimental development of an
organism and the structures that result. The actual problems are naturally
very different, since the formation is a complex of individual plants, but the
fundamental basis of habitat, function, and structure is common to both.
However, the functions now to be considered are aggregation, invasion,
competition, etc., and the structures, zones, consocies, societies,
communities, and families. The latter may properly be regarded as
adaptations called forth by the adjustment, i. e., aggregation, migration,
ecesis, etc., of the formation to the physical factors of the habitat. As
consequences of measured factors, formational adjustment and adaptation
must themselves be carefully measured and recorded. For these purposes,
the methods of quadrat and transect, of chart, photograph, and formation
herbarium are used. Invaluable as they are for any scientific inquiry into
vegetation, such methods form the very foundation of experimental study in
which accuracy is the first desideratum.
It has already been shown that nature’s own experiments in the
production of new forms furnish the best material for experimental
evolution. This statement is equally true of experimental vegetation. The
formation of new habitats by weathering and transport, and the denuding of
old ones, yield experimental plots of the greatest value. This is likewise the
case in the great majority of formations, where invasion or competition is
active. These are the phenomena that must be considered in any careful
study of vegetation, but in taking them up from the experimental standpoint,
greater attention must be paid to detail, and the changes must be followed
closely for a longer time. The method that makes use of existing changes in
vegetation is designated the method of natural habitats. In contrast with this
is the method of artificial habitats, in which the habitat itself is definitely
modified, or a group of species actually transferred to a different habitat.
Many problems of vegetation can be attacked with greater success under
control than in the field. This is particularly true of competition, in which
356. Scope and methods. The experimental study of the formation as a
complex organism rests upon methods essentially similar to those discussed
under experimental evolution. The scope of the two fields is practically the
same, moreover, in that both deal with the experimental development of an
organism and the structures that result. The actual problems are naturally
very different, since the formation is a complex of individual plants, but the
fundamental basis of habitat, function, and structure is common to both.
However, the functions now to be considered are aggregation, invasion,
competition, etc., and the structures, zones, consocies, societies,
communities, and families. The latter may properly be regarded as
adaptations called forth by the adjustment, i. e., aggregation, migration,
ecesis, etc., of the formation to the physical factors of the habitat. As
consequences of measured factors, formational adjustment and adaptation
must themselves be carefully measured and recorded. For these purposes,
the methods of quadrat and transect, of chart, photograph, and formation
herbarium are used. Invaluable as they are for any scientific inquiry into
vegetation, such methods form the very foundation of experimental study in
which accuracy is the first desideratum.
It has already been shown that nature’s own experiments in the
production of new forms furnish the best material for experimental
evolution. This statement is equally true of experimental vegetation. The
formation of new habitats by weathering and transport, and the denuding of
old ones, yield experimental plots of the greatest value. This is likewise the
case in the great majority of formations, where invasion or competition is
active. These are the phenomena that must be considered in any careful
study of vegetation, but in taking them up from the experimental standpoint,
greater attention must be paid to detail, and the changes must be followed
closely for a longer time. The method that makes use of existing changes in
vegetation is designated the method of natural habitats. In contrast with this
is the method of artificial habitats, in which the habitat itself is definitely
modified, or a group of species actually transferred to a different habitat.
Many problems of vegetation can be attacked with greater success under
control than in the field. This is particularly true of competition, in which
Page 398
results can be obtained most readily by means of the method of control
habitats, as carried on in the plant house.
habitats, as carried on in the plant house.
Page 399
METHOD OF NATURAL HABITATS
357. Natural experiments. Every family as well as every community
constitutes an experiment in competition; the same statement necessarily
holds for the larger groups, society, consocies, and formation, which are
composed of families and communities. The last also make it possible to
study competition in two typical instances, viz., in the family, where the
individuals are of one kind, and in the community, where they belong to
two or more different species. The community, moreover, is a product of
invasion, and it furnishes material for the study of this function, as well as
for that of aggregation and competition. Practically every formation shows
some invasion, but as a rule stable formations contain so few invaders that
they are relatively unimportant in this connection. Invasion is most active in
transition areas and in mixed formations, whether produced by
juxtaposition or by succession, and its study in these places yields by far the
largest number of valuable results.
As typical complete invasion, a succession is the best of all natural
experiments in aggregation, migration, ecesis, and competition. This is
especially true of the initial stages in which changes in the number and
position are most readily followed. The methods used in studying
successions have been given elsewhere. In addition, it should be pointed out
that one of the first tasks in taking up the ecological investigation of a
region is to make a careful search for all new and denuded areas, as well as
for those in which succession is taking place. The phenomena in these areas
can not be explained until the habitats and formations have been worked
over critically, but the facts must be collected at the earliest possible
moment, since the stages of the succession are constantly changing, while
the stable formations are not.
357. Natural experiments. Every family as well as every community
constitutes an experiment in competition; the same statement necessarily
holds for the larger groups, society, consocies, and formation, which are
composed of families and communities. The last also make it possible to
study competition in two typical instances, viz., in the family, where the
individuals are of one kind, and in the community, where they belong to
two or more different species. The community, moreover, is a product of
invasion, and it furnishes material for the study of this function, as well as
for that of aggregation and competition. Practically every formation shows
some invasion, but as a rule stable formations contain so few invaders that
they are relatively unimportant in this connection. Invasion is most active in
transition areas and in mixed formations, whether produced by
juxtaposition or by succession, and its study in these places yields by far the
largest number of valuable results.
As typical complete invasion, a succession is the best of all natural
experiments in aggregation, migration, ecesis, and competition. This is
especially true of the initial stages in which changes in the number and
position are most readily followed. The methods used in studying
successions have been given elsewhere. In addition, it should be pointed out
that one of the first tasks in taking up the ecological investigation of a
region is to make a careful search for all new and denuded areas, as well as
for those in which succession is taking place. The phenomena in these areas
can not be explained until the habitats and formations have been worked
over critically, but the facts must be collected at the earliest possible
moment, since the stages of the succession are constantly changing, while
the stable formations are not.
Page 400
METHOD OF ARTIFICIAL HABITATS
358. Modification of habitat. As the final factors in ecesis and
competition, water, light, and temperature control the grouping of plants
into vegetation. An efficient change in one of these, or in all of them, brings
about a visible adjustment in the structure of the plant group concerned.
Modifications of water-content and light are readily produced in the field by
drainage, irrigation, shading, clearing, etc. In fact, all the changes of habitat
indicated under experimental evolution serve equally well to initiate
experiments in experimental vegetation; indeed, the same experiment
covers both fields. It is impracticable, however, to modify the temperature
of a habitat without changing its water-content or light, and consequently
the influence of temperature can not be determined through experiment by
modification. The extent of the area modified should be as large as
convenience will permit, in order that the number of individuals may be
large enough to indicate clearly the resulting adjustment in position and
arrangement. The best results can be obtained where a small separate area
of a formation can be modified, e. g., where a small swamp can be drained,
or a depression flooded. In the case of light, however, it is usually
impossible to clear or to shade a large area, and the study must be restricted
to a relatively small group of plants. In regions where lumbering is actively
carried on, the consequent clearing initiates invaluable experiments over
large areas, and this is likewise true of forest plantations. Modification of a
large area has decided advantages in bringing out the changes in the more
prominent structural features, but the causes and the details of the
adjustment can be worked out much more satisfactorily in a small area.
359. Denuding. The modification of the habitat by denuding is the sole
method of initiating succession by experiment. It is consequently of the
most fundamental importance in investigating aggregation, ecesis, and
competition, as well as the reactions exerted by the invaders of the different
stages. The possibilities of denuding an entire habitat or an extensive area
are not great, and the investigator must content himself with denuded
quadrats, transects, and migration circles, which are small enough to permit
a critical study of all the factors in succession. It is of course unnecessary
that the denuding be done by the ecologist himself, provided he is able to
358. Modification of habitat. As the final factors in ecesis and
competition, water, light, and temperature control the grouping of plants
into vegetation. An efficient change in one of these, or in all of them, brings
about a visible adjustment in the structure of the plant group concerned.
Modifications of water-content and light are readily produced in the field by
drainage, irrigation, shading, clearing, etc. In fact, all the changes of habitat
indicated under experimental evolution serve equally well to initiate
experiments in experimental vegetation; indeed, the same experiment
covers both fields. It is impracticable, however, to modify the temperature
of a habitat without changing its water-content or light, and consequently
the influence of temperature can not be determined through experiment by
modification. The extent of the area modified should be as large as
convenience will permit, in order that the number of individuals may be
large enough to indicate clearly the resulting adjustment in position and
arrangement. The best results can be obtained where a small separate area
of a formation can be modified, e. g., where a small swamp can be drained,
or a depression flooded. In the case of light, however, it is usually
impossible to clear or to shade a large area, and the study must be restricted
to a relatively small group of plants. In regions where lumbering is actively
carried on, the consequent clearing initiates invaluable experiments over
large areas, and this is likewise true of forest plantations. Modification of a
large area has decided advantages in bringing out the changes in the more
prominent structural features, but the causes and the details of the
adjustment can be worked out much more satisfactorily in a small area.
359. Denuding. The modification of the habitat by denuding is the sole
method of initiating succession by experiment. It is consequently of the
most fundamental importance in investigating aggregation, ecesis, and
competition, as well as the reactions exerted by the invaders of the different
stages. The possibilities of denuding an entire habitat or an extensive area
are not great, and the investigator must content himself with denuded
quadrats, transects, and migration circles, which are small enough to permit
a critical study of all the factors in succession. It is of course unnecessary
that the denuding be done by the ecologist himself, provided he is able to
Page 401
follow the succession from the very beginning. Accordingly, it becomes
possible for him to make the very best use of all those changes wrought by
man in which the vegetation is destroyed over considerable areas. These are
essentially natural experiments, and at this point the methods of natural and
artificial habitats merge.
The manner of denuding depends in a degree upon the nature of
vegetation, but, when time, convenience, and safety are all taken into
account, the actual removal of the vegetation as indicated under the
denuded quadrat is by far the most satisfactory. Under certain conditions,
flooding or burning can be used to advantage, but cases of this kind are
infrequent. The purpose of the experiment determines the kind of area to be
denuded. Quadrat, transact, and migration circle are equally valuable for
ecesis and competition. The quadrat is best adapted to work in a
homogeneous area, while the transect is suited to a heterogeneous one
characterized by zones, societies, or communities. It is an advantage to
replace the denuded transect by a series of denuded quadrats, one for each
zone or society, when the transect would be too long for convenience. The
denuded migration circle is invaluable for aggregation and ecesis, since it
makes possible the study of migration as a distinct function. A series of
denuded quadrats, consisting of one or more in the different stages of a
succession, furnishes important evidence concerning the development of
each stage. By far the best method, however, for making a comparative
study of the stages of a succession is the quadrat sequence. A quadrat is
denuded each year, thus yielding a complete sequence of miniature stages
through the whole course of succession. This method is especially valuable
when a succession is represented by a single example, and there is no
opportunity of reconstructing it by the comparison of various stages. A
quadrat sequence is naturally of the greatest value if begun at the time when
the first invaders appear.
360. Modification of the formation by transfer. The study of partial
and intermittent invasion into an established vegetation is made through the
transfer of a species or group of species by means of seeding or planting.
The process differs in no way from that described for experimental
evolution, except in so far that an endeavor is made to establish a family or
a community, and not merely a few individuals. Transfer makes possible the
critical investigation of ecesis under conditions of intense competition, as
well as the study of aggregation and the origin of plant groups under these
possible for him to make the very best use of all those changes wrought by
man in which the vegetation is destroyed over considerable areas. These are
essentially natural experiments, and at this point the methods of natural and
artificial habitats merge.
The manner of denuding depends in a degree upon the nature of
vegetation, but, when time, convenience, and safety are all taken into
account, the actual removal of the vegetation as indicated under the
denuded quadrat is by far the most satisfactory. Under certain conditions,
flooding or burning can be used to advantage, but cases of this kind are
infrequent. The purpose of the experiment determines the kind of area to be
denuded. Quadrat, transact, and migration circle are equally valuable for
ecesis and competition. The quadrat is best adapted to work in a
homogeneous area, while the transect is suited to a heterogeneous one
characterized by zones, societies, or communities. It is an advantage to
replace the denuded transect by a series of denuded quadrats, one for each
zone or society, when the transect would be too long for convenience. The
denuded migration circle is invaluable for aggregation and ecesis, since it
makes possible the study of migration as a distinct function. A series of
denuded quadrats, consisting of one or more in the different stages of a
succession, furnishes important evidence concerning the development of
each stage. By far the best method, however, for making a comparative
study of the stages of a succession is the quadrat sequence. A quadrat is
denuded each year, thus yielding a complete sequence of miniature stages
through the whole course of succession. This method is especially valuable
when a succession is represented by a single example, and there is no
opportunity of reconstructing it by the comparison of various stages. A
quadrat sequence is naturally of the greatest value if begun at the time when
the first invaders appear.
360. Modification of the formation by transfer. The study of partial
and intermittent invasion into an established vegetation is made through the
transfer of a species or group of species by means of seeding or planting.
The process differs in no way from that described for experimental
evolution, except in so far that an endeavor is made to establish a family or
a community, and not merely a few individuals. Transfer makes possible the
critical investigation of ecesis under conditions of intense competition, as
well as the study of aggregation and the origin of plant groups under these
Page 402
conditions. Perhaps its greatest value is in the experimental study of
alternation and zonation, especially the former. It is practically impossible
to determine whether alternation, especially when corresponsive, is due to
physical or historical causes, i. e., migration and competition, except by
means of the reciprocal transfer of the species concerned.
Field cultures for the careful study of ecesis and competition are made by
transferring seeds or plants to new or denuded soils. This is practically a
combination of the methods of modification and transfer. It has a unique
value in making it possible to initiate artificial successions of almost any
character that is desired, and to carry them out with the reactions more or
less under control. This opens up an extremely important field of
experimental inquiry, which promises to put the study of succession upon a
much more exact basis. Competition cultures in the field are not essentially
different from those under control, and they will be considered under the
next method.
alternation and zonation, especially the former. It is practically impossible
to determine whether alternation, especially when corresponsive, is due to
physical or historical causes, i. e., migration and competition, except by
means of the reciprocal transfer of the species concerned.
Field cultures for the careful study of ecesis and competition are made by
transferring seeds or plants to new or denuded soils. This is practically a
combination of the methods of modification and transfer. It has a unique
value in making it possible to initiate artificial successions of almost any
character that is desired, and to carry them out with the reactions more or
less under control. This opens up an extremely important field of
experimental inquiry, which promises to put the study of succession upon a
much more exact basis. Competition cultures in the field are not essentially
different from those under control, and they will be considered under the
next method.
Page 403
METHOD OF CONTROL HABITATS
Fig. 83. Simple culture of floating ecads of Ranunculus sceleratus.
361. Competition cultures. Although it is quite possible to carry on
experiments in invasion and succession in the planthouse, the limited space
usually available makes this undesirable, except in a few problems where
control is necessary. Competition cultures, on the other hand, yield better
results in the planthouse than in the field, since the physical factors and the
appearance of unwelcome migrants are much more easily controlled. The
possibilities of the culture method in the study of competition seem
inexhaustible, and the author has found it necessary to confine his own
Fig. 83. Simple culture of floating ecads of Ranunculus sceleratus.
361. Competition cultures. Although it is quite possible to carry on
experiments in invasion and succession in the planthouse, the limited space
usually available makes this undesirable, except in a few problems where
control is necessary. Competition cultures, on the other hand, yield better
results in the planthouse than in the field, since the physical factors and the
appearance of unwelcome migrants are much more easily controlled. The
possibilities of the culture method in the study of competition seem
inexhaustible, and the author has found it necessary to confine his own
Page 404
investigations to a few of the fundamental problems. In this work, he has
distinguished several kinds of cultures, based chiefly upon the species
concerned and the arrangement of the individuals. Simple cultures are those
in which a single species is used. The resulting group is a family, and the
competition is between like individuals. In such cultures, the problem of the
factors in competition is reduced to its simplest terms. Mixed cultures are
based upon two or more species, and the problem is correspondingly
complicated. As a rule, all the seeds have been sown at the same time in
both simple and mixed cultures, but it has been found desirable to make
some heterochronous cultures, in which seeds are also sown after the plants
have appeared. Mixed cultures are distinguished as layered cultures, when
the species are of very different height. Thus, rosettes have been grown with
stemmed plants, tall slender forms with low branching ones, erect plants
with twining and climbing plants, etc. Further evidence as to the nature of
competition has been sought by means of ecad cultures, and factor cultures.
In the former, plants of different response to water and light are grown
together under the same conditions, in order to evaluate the part played by
the nature of the plant. In a factor culture, the area is divided into two or
more parts which are given different amounts of water or of light, in order
to determine the influence of slight variations upon the same competitors.
In somewhat similar fashion, an attempt has been made to ascertain the
bearing of biotic factors upon competition. Cultures are easily made in
which Cuscuta or parasitic fungi are used to place certain species at a
disadvantage. Permanent cultures are obtained by allowing the plants to
ripen and drop their seeds for several generations, just as in nature. They are
indispensable for determining the final outcome of the competition between
different species.
distinguished several kinds of cultures, based chiefly upon the species
concerned and the arrangement of the individuals. Simple cultures are those
in which a single species is used. The resulting group is a family, and the
competition is between like individuals. In such cultures, the problem of the
factors in competition is reduced to its simplest terms. Mixed cultures are
based upon two or more species, and the problem is correspondingly
complicated. As a rule, all the seeds have been sown at the same time in
both simple and mixed cultures, but it has been found desirable to make
some heterochronous cultures, in which seeds are also sown after the plants
have appeared. Mixed cultures are distinguished as layered cultures, when
the species are of very different height. Thus, rosettes have been grown with
stemmed plants, tall slender forms with low branching ones, erect plants
with twining and climbing plants, etc. Further evidence as to the nature of
competition has been sought by means of ecad cultures, and factor cultures.
In the former, plants of different response to water and light are grown
together under the same conditions, in order to evaluate the part played by
the nature of the plant. In a factor culture, the area is divided into two or
more parts which are given different amounts of water or of light, in order
to determine the influence of slight variations upon the same competitors.
In somewhat similar fashion, an attempt has been made to ascertain the
bearing of biotic factors upon competition. Cultures are easily made in
which Cuscuta or parasitic fungi are used to place certain species at a
disadvantage. Permanent cultures are obtained by allowing the plants to
ripen and drop their seeds for several generations, just as in nature. They are
indispensable for determining the final outcome of the competition between
different species.
Page 405
Fig. 84. Mixed culture of Solidago rigida and Onagra biennis.
362. Details of culture methods. All competition cultures have been
made 1 meter square. In other words, they are quadrats, and they are treated
exactly as denuded quadrats in the field with respect to factor readings,
charts, and photographs. In the writer’s studies, germination tests were
made of a large number of species, and those selected which showed a high
per cent of germinability. Since this was the first experimental study of
competition, this test was deemed necessary, but it is quite evident that no
such selection is made in nature. Consequently, when the seeds used are
known to be fresh, a germination test is usually superfluous. Considerable
care was taken also to select species known to be vigorous growers, with
the result that practically all the species used for experiment were ruderal or
subruderal. The species employed, and the kinds of cultures in which they
were grouped were as follows:
362. Details of culture methods. All competition cultures have been
made 1 meter square. In other words, they are quadrats, and they are treated
exactly as denuded quadrats in the field with respect to factor readings,
charts, and photographs. In the writer’s studies, germination tests were
made of a large number of species, and those selected which showed a high
per cent of germinability. Since this was the first experimental study of
competition, this test was deemed necessary, but it is quite evident that no
such selection is made in nature. Consequently, when the seeds used are
known to be fresh, a germination test is usually superfluous. Considerable
care was taken also to select species known to be vigorous growers, with
the result that practically all the species used for experiment were ruderal or
subruderal. The species employed, and the kinds of cultures in which they
were grouped were as follows:
Page 406
Fig. 85. Heterochronous culture of Helianthus annuus and Datura stramonium. Family culture of
Datura, Verbascum, etc., in the foreground.
1. Simple culture of Helianthus annuus. The culture plot was divided into four equal parts; 12
seeds were planted in one, 25 in another, 50 in the third, and 100 in the fourth.
2. Mixed culture of Helianthus annuus, Panicum virgatum, and Elymus canadensis. Twenty-five
seeds each of Helianthus and Panicum were planted alternately at equal distances in one-half of the
plot, while the other half was planted similarly with Helianthus and Elymus.
3. Mixed culture of Solidago rigida and Onagra biennis. Over one-half of the plot were scattered
50 seeds of Solidago and 100 of Onagra; over the other, 100 and 200 seeds respectively.
4. Layered culture of Laciniaria punctata, Bidens frondosa, Salvia pitcheri, Cassia chamaecrista
and Kuhnia glutinosa. Fifty seeds of each species were scattered more or less uniformly over the
entire plot.
5. Layered culture of Silphium laciniatum, Datura stramonium and Lactuca ludoviciana. Fifty
seeds of Datura and Lactuca, and 25 of Silphium were sown uniformly in one-half of the plot. In the
other half, 25 holes were made at equal intervals, and one seed of each of the three planted in each
hole.
Datura, Verbascum, etc., in the foreground.
1. Simple culture of Helianthus annuus. The culture plot was divided into four equal parts; 12
seeds were planted in one, 25 in another, 50 in the third, and 100 in the fourth.
2. Mixed culture of Helianthus annuus, Panicum virgatum, and Elymus canadensis. Twenty-five
seeds each of Helianthus and Panicum were planted alternately at equal distances in one-half of the
plot, while the other half was planted similarly with Helianthus and Elymus.
3. Mixed culture of Solidago rigida and Onagra biennis. Over one-half of the plot were scattered
50 seeds of Solidago and 100 of Onagra; over the other, 100 and 200 seeds respectively.
4. Layered culture of Laciniaria punctata, Bidens frondosa, Salvia pitcheri, Cassia chamaecrista
and Kuhnia glutinosa. Fifty seeds of each species were scattered more or less uniformly over the
entire plot.
5. Layered culture of Silphium laciniatum, Datura stramonium and Lactuca ludoviciana. Fifty
seeds of Datura and Lactuca, and 25 of Silphium were sown uniformly in one-half of the plot. In the
other half, 25 holes were made at equal intervals, and one seed of each of the three planted in each
hole.
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6. Ecad culture of Oenothera rhombipetala (xerophytic), Verbascum thapsus (mesophytic), and
Penthorum sedoides (hydrophytic). One hundred seeds of Oenothera and 200 each of Verbascum and
Penthorum were scattered over the plot.
7. Heterochronous culture of Helianthus annuus and Datura stramonium. One hundred seeds of
Helianthus were scattered over one half, and the same number of Datura seeds over the other half of
the plot. In both, also, 50 seeds were sown in one 4–inch circle, and 25 seeds in a second circle at
some distance. A month later, 100 seeds of Helianthus were sown in the Datura plot, and vice versa.
8. Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra, Datura, Penthorum, Solidago
and Verbascum. The plot was divided into 9 squares and in each were sown 50 seeds of one of these
plants.
9. Community culture. The sowing was made exactly as for the family culture, except that 20 seeds
of each plant were used. In the middle of each square, 5 seeds of a different species were planted. For
the Helianthus, Kuhnia, and Panicum groups, Onagra was used; for Bidens, Onagra, and Datura,
Helianthus was used, and for Penthorum, Solidago, and Verbascum, Panicum.
At the time the cultures were started, check plants were sown in pots. The
most vigorous seedlings were transplanted singly to large pots, and grown
under conditions of water, light, and soil as similar as possible to those of
the competition plots. Photographs of check plants and plots were made at
the proper intervals, and the plots were charted in quadrats to show the
course of competition. The factors which control competition were sought
in a critical study of water-content and light values, which is still in process.
This work has gone far enough to indicate the correctness of the view[45]
that competition is purely physical in character. It has, moreover, been
demonstrated that “room” in competition is merely a loose expression for
the relation between the number of individuals in a given space, and the
amount of water, light, and temperature available in the same space.
Penthorum sedoides (hydrophytic). One hundred seeds of Oenothera and 200 each of Verbascum and
Penthorum were scattered over the plot.
7. Heterochronous culture of Helianthus annuus and Datura stramonium. One hundred seeds of
Helianthus were scattered over one half, and the same number of Datura seeds over the other half of
the plot. In both, also, 50 seeds were sown in one 4–inch circle, and 25 seeds in a second circle at
some distance. A month later, 100 seeds of Helianthus were sown in the Datura plot, and vice versa.
8. Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra, Datura, Penthorum, Solidago
and Verbascum. The plot was divided into 9 squares and in each were sown 50 seeds of one of these
plants.
9. Community culture. The sowing was made exactly as for the family culture, except that 20 seeds
of each plant were used. In the middle of each square, 5 seeds of a different species were planted. For
the Helianthus, Kuhnia, and Panicum groups, Onagra was used; for Bidens, Onagra, and Datura,
Helianthus was used, and for Penthorum, Solidago, and Verbascum, Panicum.
At the time the cultures were started, check plants were sown in pots. The
most vigorous seedlings were transplanted singly to large pots, and grown
under conditions of water, light, and soil as similar as possible to those of
the competition plots. Photographs of check plants and plots were made at
the proper intervals, and the plots were charted in quadrats to show the
course of competition. The factors which control competition were sought
in a critical study of water-content and light values, which is still in process.
This work has gone far enough to indicate the correctness of the view[45]
that competition is purely physical in character. It has, moreover, been
demonstrated that “room” in competition is merely a loose expression for
the relation between the number of individuals in a given space, and the
amount of water, light, and temperature available in the same space.
Page 408
GLOSSARY
Note: Last terms frequent in compounds are found in their proper place
alphabetically. The accent is indicated only in those words accented on the
penult; all others are accented on the antepenult, or recessively.
abundance, the total number of individuals in an area.
acospore (ἀκή, point), a plant with awned disseminules.
acrophyti´um (ἄκρον, peak), an alpine plant formation.
acti´um (ἀκτή, rocky coast), a rocky seashore formation; actad, plant of
a rocky seashore.
-ad (-αδης, patronymic suffix), suffix for denoting an ecad.
adaptable, able to originate ecads; adaptation, the structural response to
stimuli.
adjustment, the functional response to stimuli.
adventicious (adventicius, foreign), invading from distant formations.
adventive (adventivus, accidental), established temporarily.
aggregation, the coming together of plants into groups.
agi´um (ἀγή, beach), a beach formation; agad, a beach plant.
agri´um (ἀγρός, field), a culture formation; agrad, a cultivated plant.
aiphyti´um (ἀεί, permanent), an ultimate formation.
alsi´um (ἄλσος, grove), a grove formation; alsad, a grove plant.
alternation, the heterogeneous arrangement of plant groups and
formations universally present in vegetation.
Note: Last terms frequent in compounds are found in their proper place
alphabetically. The accent is indicated only in those words accented on the
penult; all others are accented on the antepenult, or recessively.
abundance, the total number of individuals in an area.
acospore (ἀκή, point), a plant with awned disseminules.
acrophyti´um (ἄκρον, peak), an alpine plant formation.
acti´um (ἀκτή, rocky coast), a rocky seashore formation; actad, plant of
a rocky seashore.
-ad (-αδης, patronymic suffix), suffix for denoting an ecad.
adaptable, able to originate ecads; adaptation, the structural response to
stimuli.
adjustment, the functional response to stimuli.
adventicious (adventicius, foreign), invading from distant formations.
adventive (adventivus, accidental), established temporarily.
aggregation, the coming together of plants into groups.
agi´um (ἀγή, beach), a beach formation; agad, a beach plant.
agri´um (ἀγρός, field), a culture formation; agrad, a cultivated plant.
aiphyti´um (ἀεί, permanent), an ultimate formation.
alsi´um (ἄλσος, grove), a grove formation; alsad, a grove plant.
alternation, the heterogeneous arrangement of plant groups and
formations universally present in vegetation.
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amathi´um (ἄμαθος, sand of the plain), a sand-hill or sandplain
formation; amathad, a sand-hill plant.
ammochthi´um (ἄμμος, sand, ὄχθη, bank), a sand bank formation;
ammochthad, a sand bank plant.
anci´um (ἄγκος, mountain glen), a canyon formation; ancad, a canyon
plant.
anemi´um (ἄνεμος, wind), a blowout formation; anemad, a blowout
plant; anemochore, a plant distributed by wind.
-anum (locative suffix), a suffix denoting a layer.
apostrophe (ἀπό, away from, στροφή, a turning), the arrangement of the
row of chloroplasts parallel to the rays of light.
apparent noon, the time when the sun crosses the meridian, i. e., sun
noon as distinguished from noon, standard time.
-ard (ἄρδον, water of the land), combining term for water-content;
ardium, a succession due to irrigation.
ardesiacus, slate colored.
-are (locative suffix), suffix denoting a community.
aspect (aspectus, appearance), the seasonal impress of a formation, e.g.,
the spring aspect.
association, the arrangement of individuals in vegetation.
atmometer (ἀτμός, vapor), an instrument for measuring evaporation.
atropurpureus, dark purple.
atrovirens, dark green.
autochore (αὐτός, self), motile plants, or those with motile spores;
autochthonous (χθών, ground), native.
avellaneus, drab.
barrier, a physical or biological obstacle to migration or ecesis.
bathyphyti´um (βαθύς, low), a lowland plant formation.
blastochore (βλάστη, growth), a plant distributed by offshoots.
formation; amathad, a sand-hill plant.
ammochthi´um (ἄμμος, sand, ὄχθη, bank), a sand bank formation;
ammochthad, a sand bank plant.
anci´um (ἄγκος, mountain glen), a canyon formation; ancad, a canyon
plant.
anemi´um (ἄνεμος, wind), a blowout formation; anemad, a blowout
plant; anemochore, a plant distributed by wind.
-anum (locative suffix), a suffix denoting a layer.
apostrophe (ἀπό, away from, στροφή, a turning), the arrangement of the
row of chloroplasts parallel to the rays of light.
apparent noon, the time when the sun crosses the meridian, i. e., sun
noon as distinguished from noon, standard time.
-ard (ἄρδον, water of the land), combining term for water-content;
ardium, a succession due to irrigation.
ardesiacus, slate colored.
-are (locative suffix), suffix denoting a community.
aspect (aspectus, appearance), the seasonal impress of a formation, e.g.,
the spring aspect.
association, the arrangement of individuals in vegetation.
atmometer (ἀτμός, vapor), an instrument for measuring evaporation.
atropurpureus, dark purple.
atrovirens, dark green.
autochore (αὐτός, self), motile plants, or those with motile spores;
autochthonous (χθών, ground), native.
avellaneus, drab.
barrier, a physical or biological obstacle to migration or ecesis.
bathyphyti´um (βαθύς, low), a lowland plant formation.
blastochore (βλάστη, growth), a plant distributed by offshoots.
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-bole (βολή, a throw), combining term for propulsion; bolochore, a plant
distributed by propulsion.
broti´um (βροτός, mortal), a succession caused by man; brotochore, a
plant distributed by man.
caeruleus, pale blue.
caesius, eye-blue.
camni´um (κάμνω, cultivate), a succession due to cultivation.
carphospore (κάρφος, scale), a plant with disseminules possessing a
scaly or chaffy pappus.
carpostrote (καρπός, fruit), a plant migrating by means of fruits.
centrospore (κέντρον, spur), a plant with spiny disseminules.
chalici´um (χάλιξ, gravel), a gravel slide formation; chalicad, a gravel
slide plant.
cheradi´um (χέραδος, a sandbar), a wet sandbar formation; cheradad, a
wet sandbar plant.
chersi´um (χέρσος, dry barren waste), a dry waste formation; chersad,
plant of a dry waste.
chioni´um (χιών, όνος, snow), a snow formation; chionad, a snow plant;
chionophyti´um, a niveal plant formation.
chledi´um (χλῆδος, rubbish), a ruderal formation; chledad, a ruderal
plant.
chlorenchym (χλωρός, greenish yellow, ἐνχύμα, infusion), the
chlorophyll tissue of the leaf.
-chore (χωρέω, to spread abroad), combining term to denote agent of
migration.
chresard (χρῆσις, use), the available water of the soil, the physiological
water-content.
clitochore (κλίτος, slope), a plant distributed by gravity.
clysi´um (κλύσις, a flooding), a succession in a flooded soil.
distributed by propulsion.
broti´um (βροτός, mortal), a succession caused by man; brotochore, a
plant distributed by man.
caeruleus, pale blue.
caesius, eye-blue.
camni´um (κάμνω, cultivate), a succession due to cultivation.
carphospore (κάρφος, scale), a plant with disseminules possessing a
scaly or chaffy pappus.
carpostrote (καρπός, fruit), a plant migrating by means of fruits.
centrospore (κέντρον, spur), a plant with spiny disseminules.
chalici´um (χάλιξ, gravel), a gravel slide formation; chalicad, a gravel
slide plant.
cheradi´um (χέραδος, a sandbar), a wet sandbar formation; cheradad, a
wet sandbar plant.
chersi´um (χέρσος, dry barren waste), a dry waste formation; chersad,
plant of a dry waste.
chioni´um (χιών, όνος, snow), a snow formation; chionad, a snow plant;
chionophyti´um, a niveal plant formation.
chledi´um (χλῆδος, rubbish), a ruderal formation; chledad, a ruderal
plant.
chlorenchym (χλωρός, greenish yellow, ἐνχύμα, infusion), the
chlorophyll tissue of the leaf.
-chore (χωρέω, to spread abroad), combining term to denote agent of
migration.
chresard (χρῆσις, use), the available water of the soil, the physiological
water-content.
clitochore (κλίτος, slope), a plant distributed by gravity.
clysi´um (κλύσις, a flooding), a succession in a flooded soil.
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-colus (κόλος, dwelling in), combining term for habitat forms.
community, a mixture of the individuals of two or more species, a group
of families.
comospore (κόμη, hair) a plant with hairy or silky disseminules.
competition, the relation between plants occupying the same area, and
dependent upon the same supply of physical factors.
consocies, that subdivision of a formation controlled by a facies.
copious, used of species in which the individuals are arranged closely but
uniformly.
coryphi´um (κορυφή, peak), an alpine meadow formation; coryphad, an
alpine meadow plant.
creatospore (κρέας, ατος, meat), a plant with nut fruits.
cremni´um (κρημνός, crag, cliff), a cliff formation; cremnad, a cliff
plant.
creni´um (κρήνη, spring), a spring formation; crenad, a spring plant.
crymi´um (κρυμός, frost), a polar barren formation; crymad, a polar
plant; crymophytic, pertaining to polar plants.
crystallochore (κρύσταλλος, ice), a plant distributed by glaciers.
cyaneus, azure.
cyriodoche (κύριος, regular), a normal succession.
dendri´um (δένδρα, fruit trees), an orchard formation; dendrad, an
orchard plant.
derived, coming from other formations or regions, not native.
diphotic (δι-, two), the two surfaces unequally lighted; diphotophyll, a
leaf differentiated into palisade and sponge tissues owing to unequal
illumination.
diplophyll (διπλόος, twofold), an isophotic leaf with water-storage cells
in the middle.
disseminule (semen, seed), a seed fruit modified for migration.
community, a mixture of the individuals of two or more species, a group
of families.
comospore (κόμη, hair) a plant with hairy or silky disseminules.
competition, the relation between plants occupying the same area, and
dependent upon the same supply of physical factors.
consocies, that subdivision of a formation controlled by a facies.
copious, used of species in which the individuals are arranged closely but
uniformly.
coryphi´um (κορυφή, peak), an alpine meadow formation; coryphad, an
alpine meadow plant.
creatospore (κρέας, ατος, meat), a plant with nut fruits.
cremni´um (κρημνός, crag, cliff), a cliff formation; cremnad, a cliff
plant.
creni´um (κρήνη, spring), a spring formation; crenad, a spring plant.
crymi´um (κρυμός, frost), a polar barren formation; crymad, a polar
plant; crymophytic, pertaining to polar plants.
crystallochore (κρύσταλλος, ice), a plant distributed by glaciers.
cyaneus, azure.
cyriodoche (κύριος, regular), a normal succession.
dendri´um (δένδρα, fruit trees), an orchard formation; dendrad, an
orchard plant.
derived, coming from other formations or regions, not native.
diphotic (δι-, two), the two surfaces unequally lighted; diphotophyll, a
leaf differentiated into palisade and sponge tissues owing to unequal
illumination.
diplophyll (διπλόος, twofold), an isophotic leaf with water-storage cells
in the middle.
disseminule (semen, seed), a seed fruit modified for migration.
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dissophyte (δισσός, double), a plant with xerophytic leaves and stems,
and mesophytic roots.
-doche (δοχή, succession), succession.
drimi´um (δριμύς, biting, pungent), an alkaline habitat, and the
corresponding formation; drimad, a plant of such a formation.
driodi´um (δρίος, thicket), a dry thicket formation; driodad, plant of a
dry thicket.
dysgeogenous (δυς-, bad, γῆ, soil), weathering with difficulty to form
soil.
ecad (οἶκος, home), a habitat form due to origin by adaptation; ece´sis
(οἰκῆσις, act of coming to be at home), the germination and
establishment of invaders; ecograph, an instrument for measuring a
physical factor of a habitat; ecotone (τόνος, tension), the tension line
between two zones, formations, consocies, etc.
ecballi´um, (ἐκβάλλω, cut down forests), a succession due to lumbering.
echard (ἔχω, to withhold), the non-available water of the soil.
edobole (οἶδος, swelling), a plant whose seeds are scattered by
propulsion through turgescence.
efficient difference, the amount of a physical factor necessary to produce
a change in the response.
enauli´um (ἔναυλος, hollow channel), a sanddraw formation; enaulad, a
sanddraw plant.
ende´mic (ἐν, within, δῆμος, district), occurring in a single formation, or
natural region; ende´mism, the condition of growing in but one natural
area.
epistrophe (ἐπί, towards, στροφή, a turning), the arrangement of the row
of chloroplasts at right angles to the incident light.
eremi´um (ἔρημος, desert), a desert formation; eremad, a desert plant.
estival, pertaining to summer.
-etum (locative suffix), suffix used to denote a consocies.
and mesophytic roots.
-doche (δοχή, succession), succession.
drimi´um (δριμύς, biting, pungent), an alkaline habitat, and the
corresponding formation; drimad, a plant of such a formation.
driodi´um (δρίος, thicket), a dry thicket formation; driodad, plant of a
dry thicket.
dysgeogenous (δυς-, bad, γῆ, soil), weathering with difficulty to form
soil.
ecad (οἶκος, home), a habitat form due to origin by adaptation; ece´sis
(οἰκῆσις, act of coming to be at home), the germination and
establishment of invaders; ecograph, an instrument for measuring a
physical factor of a habitat; ecotone (τόνος, tension), the tension line
between two zones, formations, consocies, etc.
ecballi´um, (ἐκβάλλω, cut down forests), a succession due to lumbering.
echard (ἔχω, to withhold), the non-available water of the soil.
edobole (οἶδος, swelling), a plant whose seeds are scattered by
propulsion through turgescence.
efficient difference, the amount of a physical factor necessary to produce
a change in the response.
enauli´um (ἔναυλος, hollow channel), a sanddraw formation; enaulad, a
sanddraw plant.
ende´mic (ἐν, within, δῆμος, district), occurring in a single formation, or
natural region; ende´mism, the condition of growing in but one natural
area.
epistrophe (ἐπί, towards, στροφή, a turning), the arrangement of the row
of chloroplasts at right angles to the incident light.
eremi´um (ἔρημος, desert), a desert formation; eremad, a desert plant.
estival, pertaining to summer.
-etum (locative suffix), suffix used to denote a consocies.
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eugeogenous (εὖ-, well, γῆ, soil), weathering readily to form soil.
facies, a dominant species of a formation: a distinct area controlled by it
is a consocies.
family, a group of individuals belonging to one species.
fixity, the condition characterized by little or no response to stimuli.
flavovirens, yellow green.
forewold, equivalent to the German “vorwald,” the thicket zone
bordering a forest.
-genous (γένω, to produce), producing.
geotome (γῆ, earth, τομή, edge), an instrument for obtaining soil samples.
gloeospore (γλοιός, sticky stuff), a plant with viscid disseminules.
-graph (γραφή, a writing), combining term for a recording instrument.
gregarious (gregarius, grouped in herds), used of species in which the
individuals occur in groups.
habitat, a definite physical area characterized by a formation; habitat
form, the impress given the plant by the habitat.
harmosis (ἅρμοσις, an adapting), response to stimuli, comprising both
adjustment and adaptation.
hedi´um (ἕδος, a sitting, base), a succession in a residuary soil.
heliad (ἥλιος, sun), a heliophyte; heliophyll, the leaf of a sun plant;
heliophyte, a sun plant; heliophyti´um, a sun plant formation;
heliophilous, sun-loving.
heli´um (ἕλος, marsh), a marsh formation; helad, a marsh plant; helodi
´um (ἑλώδης, marshy), a swampy open woodland formation; helodad,
a marsh plant; helodrium (δρίος, thicket), a thicket formation;
helodrad, a plant of a marshy thicket; helohyli´um (ὕλη, forest) a
marsh forest formation; helohylad, a marsh forest plant.
hepodoche (ἕπω, follow), a secondary succession.
hizometer (ἵζω, to sink), an instrument for measuring gravitation water.
facies, a dominant species of a formation: a distinct area controlled by it
is a consocies.
family, a group of individuals belonging to one species.
fixity, the condition characterized by little or no response to stimuli.
flavovirens, yellow green.
forewold, equivalent to the German “vorwald,” the thicket zone
bordering a forest.
-genous (γένω, to produce), producing.
geotome (γῆ, earth, τομή, edge), an instrument for obtaining soil samples.
gloeospore (γλοιός, sticky stuff), a plant with viscid disseminules.
-graph (γραφή, a writing), combining term for a recording instrument.
gregarious (gregarius, grouped in herds), used of species in which the
individuals occur in groups.
habitat, a definite physical area characterized by a formation; habitat
form, the impress given the plant by the habitat.
harmosis (ἅρμοσις, an adapting), response to stimuli, comprising both
adjustment and adaptation.
hedi´um (ἕδος, a sitting, base), a succession in a residuary soil.
heliad (ἥλιος, sun), a heliophyte; heliophyll, the leaf of a sun plant;
heliophyte, a sun plant; heliophyti´um, a sun plant formation;
heliophilous, sun-loving.
heli´um (ἕλος, marsh), a marsh formation; helad, a marsh plant; helodi
´um (ἑλώδης, marshy), a swampy open woodland formation; helodad,
a marsh plant; helodrium (δρίος, thicket), a thicket formation;
helodrad, a plant of a marshy thicket; helohyli´um (ὕλη, forest) a
marsh forest formation; helohylad, a marsh forest plant.
hepodoche (ἕπω, follow), a secondary succession.
hizometer (ἵζω, to sink), an instrument for measuring gravitation water.
Page 414
holard (ὅλος, whole), the total water-content of the soil.
hydrad (ὑδρο-, water), a hydrophyte; hydrochore, a plant distributed by
water; hydroharmose, response to water stimuli; hydrophyll, the leaf
of a hydrophyte; hydrophyte, a water plant; hydrophyti´um, a water
plant formation; hydrophilous, water-loving; hydrosta´tic (στατικός,
standing), completing the succession under hydrophytic conditions;
hydrotropic (τροπικός, turning), applied to successions which become
mesophytic.
hygrome´tric (ὑγρίς, wet), measuring or absorbing water; hygroscopic
(σκοπέω, look), measurable only by a hygroscope; able to absorb
moisture.
hyli´um (ὕλη, forest), a forest formation; hylad, a forest plant;
hylocolum, dwelling in a forest; hylodi´um (ὑλώδης, wooded), a dry
open woodland formation; hylodad, a plant of this formation;
hylophyte, a forest plant.
hypsi´um (ὕψος, elevation), a succession caused by elevation.
-ile (locative affix), suffix denoting a society.
immobile, without effective devices for migration.
indigenous (indigena, sprung from the land), native.
insolation, exposure to intense heat and light.
isabellinus, leather-colored.
isolation, separation by barriers.
isopho´tic (ἴσος, equal), equally illuminated; isophotophyll, a leaf in
which both halves of the chlorenchym are alike, due to equal
illumination.
-ium (-εῖον, locative affix), suffix denoting a formation.
labile, plastic, easily modified.
lauri´um (λαύρα, drain), a drain formation; laurad, a drain plant.
limni´um (λίμνη, lake), a lake formation; limnad, a lake plant;
limnodium (λιμνῶδες, marshy ground), a salt marsh formation;
limnodad, a plant of a salt marsh.
hydrad (ὑδρο-, water), a hydrophyte; hydrochore, a plant distributed by
water; hydroharmose, response to water stimuli; hydrophyll, the leaf
of a hydrophyte; hydrophyte, a water plant; hydrophyti´um, a water
plant formation; hydrophilous, water-loving; hydrosta´tic (στατικός,
standing), completing the succession under hydrophytic conditions;
hydrotropic (τροπικός, turning), applied to successions which become
mesophytic.
hygrome´tric (ὑγρίς, wet), measuring or absorbing water; hygroscopic
(σκοπέω, look), measurable only by a hygroscope; able to absorb
moisture.
hyli´um (ὕλη, forest), a forest formation; hylad, a forest plant;
hylocolum, dwelling in a forest; hylodi´um (ὑλώδης, wooded), a dry
open woodland formation; hylodad, a plant of this formation;
hylophyte, a forest plant.
hypsi´um (ὕψος, elevation), a succession caused by elevation.
-ile (locative affix), suffix denoting a society.
immobile, without effective devices for migration.
indigenous (indigena, sprung from the land), native.
insolation, exposure to intense heat and light.
isabellinus, leather-colored.
isolation, separation by barriers.
isopho´tic (ἴσος, equal), equally illuminated; isophotophyll, a leaf in
which both halves of the chlorenchym are alike, due to equal
illumination.
-ium (-εῖον, locative affix), suffix denoting a formation.
labile, plastic, easily modified.
lauri´um (λαύρα, drain), a drain formation; laurad, a drain plant.
limni´um (λίμνη, lake), a lake formation; limnad, a lake plant;
limnodium (λιμνῶδες, marshy ground), a salt marsh formation;
limnodad, a plant of a salt marsh.
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lochmi´um (λόχμη, thicket), a thicket formation; lochmad, a thicket
plant.
lophi´um (λόφος, crest, hill), a hill formation; lophad, a hill plant;
lophospore, a plant with plumose disseminules.
mastigospore (μάστιξ, ιγος, lash), a plant with ciliate or flagellate
disseminules.
melleus, honey-colored.
meridian, used chiefly as a synonym for apparent noon; also an
imaginary line of longitude.
mesad (μέσος, middle), a mesophyte; mesophilous, growing in moist
soils; mesophyll, the leaf of a mesophyte; mesophyte, a plant of moist
soils; mesophyti´um, a mesophytic formation; mesosta´tic (στατικίς,
standing), completing the succession under mesophytic conditions;
mesotro´pic (τροπικός, turning), applied to successions which become
mesophytic.
-meter (μέτρον, measure), combining term for instrument.
micti´um (μικτόν, mixture), a mixed formation.
migrant, a plant that is migrating or invading.
migration (migratio, removal), the movement of plants into new areas;
migration circle, a circle employed to measure migration.
mobile, able to be moved, i. e., modified for migration.
monochronic (μόνος, single, χρόνος, time), arising but once;
monogenesis (γένεσις, origin), the origin of a new form at a single
place or time; monophyle´sis (φῦλον, race), origin from a single
ancestral type; monoto´pic (τόπος, place), arising at one place only.
motile, able to move by growth, by means of cilia, etc.
mutable, able to produce mutants; mutant, a form arising by mutation;
mutation, the sudden appearance of new forms.
namati´um (νάμα, ατος, brook), a brook formation; namatad, a brook
plant.
nomi´um (νομός, pasture), a pasture formation; nomad, a pasture plant.
plant.
lophi´um (λόφος, crest, hill), a hill formation; lophad, a hill plant;
lophospore, a plant with plumose disseminules.
mastigospore (μάστιξ, ιγος, lash), a plant with ciliate or flagellate
disseminules.
melleus, honey-colored.
meridian, used chiefly as a synonym for apparent noon; also an
imaginary line of longitude.
mesad (μέσος, middle), a mesophyte; mesophilous, growing in moist
soils; mesophyll, the leaf of a mesophyte; mesophyte, a plant of moist
soils; mesophyti´um, a mesophytic formation; mesosta´tic (στατικίς,
standing), completing the succession under mesophytic conditions;
mesotro´pic (τροπικός, turning), applied to successions which become
mesophytic.
-meter (μέτρον, measure), combining term for instrument.
micti´um (μικτόν, mixture), a mixed formation.
migrant, a plant that is migrating or invading.
migration (migratio, removal), the movement of plants into new areas;
migration circle, a circle employed to measure migration.
mobile, able to be moved, i. e., modified for migration.
monochronic (μόνος, single, χρόνος, time), arising but once;
monogenesis (γένεσις, origin), the origin of a new form at a single
place or time; monophyle´sis (φῦλον, race), origin from a single
ancestral type; monoto´pic (τόπος, place), arising at one place only.
motile, able to move by growth, by means of cilia, etc.
mutable, able to produce mutants; mutant, a form arising by mutation;
mutation, the sudden appearance of new forms.
namati´um (νάμα, ατος, brook), a brook formation; namatad, a brook
plant.
nomi´um (νομός, pasture), a pasture formation; nomad, a pasture plant.
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occupation, possession of the ground by plants.
oceani´um (ὠκεανός, ocean), an ocean formation; oceanad, an ocean
plant; oceanophyte, an ocean plant; oceanophilous, ocean-dwelling.
ocheti´um (ὀχετός, drain), a succession due to artificial drainage.
ochroleucus, yellowish white.
ochthi´um (ὄχθη, bank), a bank formation; ochthad, a bank plant.
oligope´lic (ὀλίγος, little, πηλός clay), containing little clay; oligopsam
´mic (ψάμμος, sand), containing little sand.
olisthi´um (ὄλισθος, slip), a succession in a landslip.
ombrometer (ὄμβρος, a rainstorm), a rain gauge.
-on (-ών, locative suffix), suffix used to denote a family.
oncospore (ὄγκος, hook), a plant with hooked disseminules.
orgadi´um (ὀργάς, άδος, meadowland partially wooded), an open
woodland formation; orgadad, an open woodland plant.
orophyti´um (ὄρος, mountain), a subalpine plant formation.
oxodi´um (ὀξώδης, sour), a humus marsh formation; oxodad, a plant of a
humus marsh.
pagi´um (πάγος, rocky hill, glacier), a succession in a glacial soil;
pagophyti´um, a foot-hill plant formation.
pediophyti´um (πεδίον, plain), an upland plant formation.
pelagi´um (πέλαγος, surface of the sea), a surface sea formation;
pelagad, a plant of the sea surface.
pelochthi´um (πηλός, mud, ὄχθη, bank), a mud bank form; pelogenous,
producing clay; pelopsammic (ψάμμος, sand), composed of mixed clay
and sand; pelopsammogenous, producing clay and sand.
permobile, extremely mobile.
perquadrat, a quadrat of 16 square meters or more.
petasospore (πέτασος, sunshade), a plant with parachute-like
disseminules.
oceani´um (ὠκεανός, ocean), an ocean formation; oceanad, an ocean
plant; oceanophyte, an ocean plant; oceanophilous, ocean-dwelling.
ocheti´um (ὀχετός, drain), a succession due to artificial drainage.
ochroleucus, yellowish white.
ochthi´um (ὄχθη, bank), a bank formation; ochthad, a bank plant.
oligope´lic (ὀλίγος, little, πηλός clay), containing little clay; oligopsam
´mic (ψάμμος, sand), containing little sand.
olisthi´um (ὄλισθος, slip), a succession in a landslip.
ombrometer (ὄμβρος, a rainstorm), a rain gauge.
-on (-ών, locative suffix), suffix used to denote a family.
oncospore (ὄγκος, hook), a plant with hooked disseminules.
orgadi´um (ὀργάς, άδος, meadowland partially wooded), an open
woodland formation; orgadad, an open woodland plant.
orophyti´um (ὄρος, mountain), a subalpine plant formation.
oxodi´um (ὀξώδης, sour), a humus marsh formation; oxodad, a plant of a
humus marsh.
pagi´um (πάγος, rocky hill, glacier), a succession in a glacial soil;
pagophyti´um, a foot-hill plant formation.
pediophyti´um (πεδίον, plain), an upland plant formation.
pelagi´um (πέλαγος, surface of the sea), a surface sea formation;
pelagad, a plant of the sea surface.
pelochthi´um (πηλός, mud, ὄχθη, bank), a mud bank form; pelogenous,
producing clay; pelopsammic (ψάμμος, sand), composed of mixed clay
and sand; pelopsammogenous, producing clay and sand.
permobile, extremely mobile.
perquadrat, a quadrat of 16 square meters or more.
petasospore (πέτασος, sunshade), a plant with parachute-like
disseminules.
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petri´um (πέτρα, rock, stone), a rock formation; petrad, a rock plant;
petrochthi´um (ὄχθη, bank), a rock bank formation.
petrodi´um (πετρώδης, abounding in boulders), a boulder field
formation; petrodad, a plant of a boulder field.
phelli´um (φελλεύς, stony ground), a rock field formation; phellad, a
rock field plant.
-philous (φίλος), loving, dwelling in.
-photic (φῶς, φωτός, light), pertaining to light; photoharmose, response
to light stimuli; photometer, an instrument for measuring light.
phreti´um (φρητός, tank), a tank formation; phretad, a tank plant.
phyad (φυή, form of growth), a vegetation form, e. g., tree, shrub, etc.
-phyll (φύλλον, leaf), combining term for leaf.
-phyte (φυτόν, plant), combining term denoting plant; phyteris (ἔρις,
strife), plant competition; -phyti´um (φυτεῖον, place covered with
plants), combining term for formation; phytostrote, a species migrating
by means of the plant body.
pladobole (πλάδος, moisture), a plant whose seeds are scattered by
propulsion due to moisture.
plasticity, the condition characterized by ready response to stimuli.
pnoi´um (πνοή, blast), a succession in an aeolian soil.
poi´um (πόα, meadow), meadow formation; poad, a meadow plant;
poophyte, a meadow plant.
polyan´thous (πολύς, many, ἄνθος, flower), producing many flowers;
polychro´nic (χρόνος, time), arising at two or more times; polyde´mic
(δῆμος, district), occurring in two or more formations or natural
regions; polygenesis (γένεσις, origin), the origin of a new form at two
or more places or times; polyphyle´sis (φῦλον, race), the origin of a
form, species, or genus from two or more ancestral types;
polyspermatous (σπέρμα, seed), producing many seeds in each flower;
polyto´pic (τόπος, place), arising at two or more distinct places.
ponti´um (πόντος, deep sea), a deep sea formation.
petrochthi´um (ὄχθη, bank), a rock bank formation.
petrodi´um (πετρώδης, abounding in boulders), a boulder field
formation; petrodad, a plant of a boulder field.
phelli´um (φελλεύς, stony ground), a rock field formation; phellad, a
rock field plant.
-philous (φίλος), loving, dwelling in.
-photic (φῶς, φωτός, light), pertaining to light; photoharmose, response
to light stimuli; photometer, an instrument for measuring light.
phreti´um (φρητός, tank), a tank formation; phretad, a tank plant.
phyad (φυή, form of growth), a vegetation form, e. g., tree, shrub, etc.
-phyll (φύλλον, leaf), combining term for leaf.
-phyte (φυτόν, plant), combining term denoting plant; phyteris (ἔρις,
strife), plant competition; -phyti´um (φυτεῖον, place covered with
plants), combining term for formation; phytostrote, a species migrating
by means of the plant body.
pladobole (πλάδος, moisture), a plant whose seeds are scattered by
propulsion due to moisture.
plasticity, the condition characterized by ready response to stimuli.
pnoi´um (πνοή, blast), a succession in an aeolian soil.
poi´um (πόα, meadow), meadow formation; poad, a meadow plant;
poophyte, a meadow plant.
polyan´thous (πολύς, many, ἄνθος, flower), producing many flowers;
polychro´nic (χρόνος, time), arising at two or more times; polyde´mic
(δῆμος, district), occurring in two or more formations or natural
regions; polygenesis (γένεσις, origin), the origin of a new form at two
or more places or times; polyphyle´sis (φῦλον, race), the origin of a
form, species, or genus from two or more ancestral types;
polyspermatous (σπέρμα, seed), producing many seeds in each flower;
polyto´pic (τόπος, place), arising at two or more distinct places.
ponti´um (πόντος, deep sea), a deep sea formation.
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potami´um (ποταμός, river), a river formation; potamad, a river plant.
potometer (ποτόν, drink), an instrument for measuring absorption.
prevernal, pertaining to early spring.
prior, earlier, used of alpine aspects.
prochosi´um (πρόχωσις, a deposition of mud), a succession in an alluvial
soil.
prodophyti´um (πρόοδος, pioneer), an initial formation.
protodoche (πρῶτος, first), a primary succession.
proximity (proximitas, nearness), nearness to the area invaded.
psamathi´um (ψάμαθος, sand of the seashore), a strand formation;
psamathad, a strand plant; psammogenous (ψάμμος, sand), producing
a sandy soil.
psili´um (ψιλά, land without trees), a prairie formation; psilad, a prairie
plant.
psychrometer (ψυχρός, chill), an instrument that measures humidity by
means of a fall in temperature; psychrograph, a psychrometer that
records automatically.
ptenophyti´um (πτηνός, passing), an intermediate formation.
pterospore (πτερόν, wing), a plant with winged disseminules.
purpureus, purple.
pycnophyti´um (πυκνός, thick), a closed formation.
pyri´um (πῦρ, fire), a burn succession.
quadrat (quadratum, a square), a square meter of vegetation marked off
for counting, mapping, etc.; major, a quadrat of 2–14 square meters.
reaction, the effect of the formation upon the habitat.
relict (relictus, left), a species belonging properly to an earlier type of
succession than the one in which it is found.
repi´um (ῥέπω, sink), a succession due to subsidence.
potometer (ποτόν, drink), an instrument for measuring absorption.
prevernal, pertaining to early spring.
prior, earlier, used of alpine aspects.
prochosi´um (πρόχωσις, a deposition of mud), a succession in an alluvial
soil.
prodophyti´um (πρόοδος, pioneer), an initial formation.
protodoche (πρῶτος, first), a primary succession.
proximity (proximitas, nearness), nearness to the area invaded.
psamathi´um (ψάμαθος, sand of the seashore), a strand formation;
psamathad, a strand plant; psammogenous (ψάμμος, sand), producing
a sandy soil.
psili´um (ψιλά, land without trees), a prairie formation; psilad, a prairie
plant.
psychrometer (ψυχρός, chill), an instrument that measures humidity by
means of a fall in temperature; psychrograph, a psychrometer that
records automatically.
ptenophyti´um (πτηνός, passing), an intermediate formation.
pterospore (πτερόν, wing), a plant with winged disseminules.
purpureus, purple.
pycnophyti´um (πυκνός, thick), a closed formation.
pyri´um (πῦρ, fire), a burn succession.
quadrat (quadratum, a square), a square meter of vegetation marked off
for counting, mapping, etc.; major, a quadrat of 2–14 square meters.
reaction, the effect of the formation upon the habitat.
relict (relictus, left), a species belonging properly to an earlier type of
succession than the one in which it is found.
repi´um (ῥέπω, sink), a succession due to subsidence.
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rhoi´um (ῥόος, stream), a creek formation; rhoad, a creek plant.
rhoptometer (ῥοπτόν, something absorbed), an instrument to measure
absorption of water by the soil.
rhyaci´um (ῥύαξ, ακος, mountain torrent), a torrent formation; rhyacad,
a torrent plant.
rhysi´um (ῥυσίς, a flowing of fire), a succession due to volcanic action.
ruber, red.
saccospore (σάκκος, sack), a plant with sack-like disseminules.
sarcospore (σάρξ, σαρκός, flesh), a plant with fleshy disseminules.
sciad (σκιά, shade), a sciophyte; sciophyll, the leaf of a shade plant;
sciophyte, a shade plant; sciophyti´um, a shade plant formation;
sciophilous, shade-loving.
selagraph (σέλας, light), an instrument for recording light values
automatically.
serotinal, late, pertaining to autumn.
social, used of plants in which the individuals are compactly grouped;
exclusive, excluding individuals of other species; inclusive, permitting
the entrance of individuals of other species.
society, a subdivision of the formation, characterized by a principal
species.
sparse, scattered singly.
spermatostrote (σπέρμα, ατος, seed), a plant migrating by means of
seeds.
sphyri´um (σφύρον, ankle, talus), a succession in a talus soil.
spongophyll (σπόγγος, a sponge), a leaf consisting of sponge tissue.
sporadophyti´um (σποράς, άδος, scattered), an open formation.
-spore (σπορά, seed, fruit), combining term for migration contrivance;
sporostrote, a plant migrating by means of spores.
stability, the condition in which the plant makes little or no response.
rhoptometer (ῥοπτόν, something absorbed), an instrument to measure
absorption of water by the soil.
rhyaci´um (ῥύαξ, ακος, mountain torrent), a torrent formation; rhyacad,
a torrent plant.
rhysi´um (ῥυσίς, a flowing of fire), a succession due to volcanic action.
ruber, red.
saccospore (σάκκος, sack), a plant with sack-like disseminules.
sarcospore (σάρξ, σαρκός, flesh), a plant with fleshy disseminules.
sciad (σκιά, shade), a sciophyte; sciophyll, the leaf of a shade plant;
sciophyte, a shade plant; sciophyti´um, a shade plant formation;
sciophilous, shade-loving.
selagraph (σέλας, light), an instrument for recording light values
automatically.
serotinal, late, pertaining to autumn.
social, used of plants in which the individuals are compactly grouped;
exclusive, excluding individuals of other species; inclusive, permitting
the entrance of individuals of other species.
society, a subdivision of the formation, characterized by a principal
species.
sparse, scattered singly.
spermatostrote (σπέρμα, ατος, seed), a plant migrating by means of
seeds.
sphyri´um (σφύρον, ankle, talus), a succession in a talus soil.
spongophyll (σπόγγος, a sponge), a leaf consisting of sponge tissue.
sporadophyti´um (σποράς, άδος, scattered), an open formation.
-spore (σπορά, seed, fruit), combining term for migration contrivance;
sporostrote, a plant migrating by means of spores.
stability, the condition in which the plant makes little or no response.
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stabilization, the tendency typical of succession, in which the successive
stages become more stable.
stasi´um (στάσις, a standing), a stagnant pool formation; stasad, a plant
of stagnant water.
staurophyll (σταυρός, a pale), a leaf consisting of palisade tissue.
sterrhi´um (στερρός, barren), a moor formation; sterrhad, a moor plant.
-strote (στρώτος, strewn), combining term for means of migration.
subcopious, scattered somewhat loosely.
subgregarious, arranged in loose groups.
subquadrat, a quadrat of 1–8 decimeters.
succession, complete and continuous or repeated invasion, in
consequence of which formations succeed each other.
symmetry, used of topography when it shows uniform changes; radial, a
condition in which the different areas are concentric; bilateral, where
the areas occur in two similar rows.
syrtidi´um (σύρτις, ιδος, sandbar), a dry sandbar formation; syrtidad, a
plant of a dry sandbar.
taphri´um (τάφρος, ditch), a ditch formation; taphrad, a ditch plant.
telmati´um (τέλμα, ατος, water meads), a wet meadow formation;
telmatad, a wet meadow plant.
testaceus, pale brick colored.
thalassi´um (θάλασσα, sea), a sea formation; thalassad, a sea plant.
thallostrote (θαλλός, shoot), a species migrating by means of offshoots.
theri´um (θήρ, wild animal), a succession due to animals.
thermi´um (θέρμη, hot spring), a hot spring formation; thermad, a hot
spring plant.
thini´um (θίς, θινός, a dune), a dune formation; thinad, a dune plant.
tiphi´um (τῖφος, pool), a pool formation; tiphad, a pond plant.
stages become more stable.
stasi´um (στάσις, a standing), a stagnant pool formation; stasad, a plant
of stagnant water.
staurophyll (σταυρός, a pale), a leaf consisting of palisade tissue.
sterrhi´um (στερρός, barren), a moor formation; sterrhad, a moor plant.
-strote (στρώτος, strewn), combining term for means of migration.
subcopious, scattered somewhat loosely.
subgregarious, arranged in loose groups.
subquadrat, a quadrat of 1–8 decimeters.
succession, complete and continuous or repeated invasion, in
consequence of which formations succeed each other.
symmetry, used of topography when it shows uniform changes; radial, a
condition in which the different areas are concentric; bilateral, where
the areas occur in two similar rows.
syrtidi´um (σύρτις, ιδος, sandbar), a dry sandbar formation; syrtidad, a
plant of a dry sandbar.
taphri´um (τάφρος, ditch), a ditch formation; taphrad, a ditch plant.
telmati´um (τέλμα, ατος, water meads), a wet meadow formation;
telmatad, a wet meadow plant.
testaceus, pale brick colored.
thalassi´um (θάλασσα, sea), a sea formation; thalassad, a sea plant.
thallostrote (θαλλός, shoot), a species migrating by means of offshoots.
theri´um (θήρ, wild animal), a succession due to animals.
thermi´um (θέρμη, hot spring), a hot spring formation; thermad, a hot
spring plant.
thini´um (θίς, θινός, a dune), a dune formation; thinad, a dune plant.
tiphi´um (τῖφος, pool), a pool formation; tiphad, a pond plant.
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tiri´um (τείρω, rub away), a bad land formation; tirad, a bad land plant.
tonobole (τόνος, tension), a plant whose seeds are scattered by projection
from calyx or involucre.
transect (transectus, cut through), a cross section of vegetation.
trechometer (τρέχω, to run off), an instrument for measuring run-off.
tribi´um (τρίβω, wear or rub away), a succession in an eroded soil.
umbrinus, umber.
variable, able to produce variants; variant, a form arising from origin by
variation; variation, the origin of new forms by the action of selection
upon minute differences.
vegetation form, a characteristic plant form, e. g., tree, rosette, etc.
vernal, pertaining to spring.
vicine (vicinus, neighboring), invading from adjacent formations.
viridis, green.
vixgregarious, arranged in small or indistinct groups.
water-content, the water of the soil or habitat; physiological, the
available soil water; physical, the total amount of soil water.
xenodoche (ξένος, strange), an anomalous succession.
xerad (ξηρός, dry), a xerophyte; xerasi´um (ξηρασία, drought), a
succession due to drainage or drought; xeriobole (ξηρία, dryness), a
plant whose seeds are scattered by dehiscence due to dryness; xerohyli
´um (ὕλη, forest), a dry forest formation; xerohylad, a dry forest plant;
xerophyll, the leaf of a xerophyte; xerophyte, a dry soil plant;
xerophyti´um, a xerophytic formation; xerophilous, dwelling in a dry
habitat; xeropoi´um, a heath formation; xeropoad, a heath plant;
xerosta´tic (στατικός, standing), used of successions which are
completed under xerophytic conditions; xerotro´pic (τροπικός,
turning), applied to successions which become xerophytic.
zonation, that condition in which plant groups or formations appear in
belts or zones.
tonobole (τόνος, tension), a plant whose seeds are scattered by projection
from calyx or involucre.
transect (transectus, cut through), a cross section of vegetation.
trechometer (τρέχω, to run off), an instrument for measuring run-off.
tribi´um (τρίβω, wear or rub away), a succession in an eroded soil.
umbrinus, umber.
variable, able to produce variants; variant, a form arising from origin by
variation; variation, the origin of new forms by the action of selection
upon minute differences.
vegetation form, a characteristic plant form, e. g., tree, rosette, etc.
vernal, pertaining to spring.
vicine (vicinus, neighboring), invading from adjacent formations.
viridis, green.
vixgregarious, arranged in small or indistinct groups.
water-content, the water of the soil or habitat; physiological, the
available soil water; physical, the total amount of soil water.
xenodoche (ξένος, strange), an anomalous succession.
xerad (ξηρός, dry), a xerophyte; xerasi´um (ξηρασία, drought), a
succession due to drainage or drought; xeriobole (ξηρία, dryness), a
plant whose seeds are scattered by dehiscence due to dryness; xerohyli
´um (ὕλη, forest), a dry forest formation; xerohylad, a dry forest plant;
xerophyll, the leaf of a xerophyte; xerophyte, a dry soil plant;
xerophyti´um, a xerophytic formation; xerophilous, dwelling in a dry
habitat; xeropoi´um, a heath formation; xeropoad, a heath plant;
xerosta´tic (στατικός, standing), used of successions which are
completed under xerophytic conditions; xerotro´pic (τροπικός,
turning), applied to successions which become xerophytic.
zonation, that condition in which plant groups or formations appear in
belts or zones.
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zone, a belt of more or less uniform vegetation.
zoochore (ζῶον, animal), a plant distributed by animals.
zoochore (ζῶον, animal), a plant distributed by animals.
Page 423
BIBLIOGRAPHY
Ascherson, P.
Von der Verbreitung der Pflanzen. Leunis Synopsis, 1:724. 1883.
Bachmann, E. T.
Darstellung der Entwicklungsgeschichte und des Baues der Samenschale
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Instructions to Field Parties and Descriptions of Soil Types. 1903.
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Biologie der Pflanzen. 1889.
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Allgemeine Bemerkungen über den Unterschied der Vegetation auf der
nördlichen und südlichen Halbkugel der Erde. 1811.
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Grundzüge der Pflanzenverbreitung auf der Iberischen Halbinsel. 1896.
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Die Einwirkung des Menschen auf die Natur. Engler Jahrb., 24:69. 1894.
Zimmer.
Ueber Pflanzenwanderungen. 1871.
1. Bonnier, G.
Les Plantes Arctiques Comparées aux Mêmes Espèces des Alps et des Pyrénées. Rev. Gen.
Bot. 6:505. 1894.
Cultures Expérimentales dans les Alps et les Pyrénées. Rev. Gen. Bot. 2:514. 1890.
Recherches Expérimentales sur l’Adaptation des Plantes au Climat Alpin. Ann. Nat. Sci.
7:20:218. 1895.
Bonnier, G., et Ch. Flahault
Modifications des végétaux sur l’influence des conditions physiques du milieu. Ann. Nat.
Sci. 6:7:93. 1878.
2. Gardner, F. D. The Electrical Methods of Moisture Determination in Soils. Bull. Div. Soils,
12:12. 1898.
Page 437
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Wiesner, J.
Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete. Sitzb. Akad. Wiss.
Wien., I, 1893. II, 1895.
Untersuchungen über das photochemische Klima von Wien, Cairo, und Buitenzorg (Java)
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Wien., 109. 1900.
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6. Meyen, F. J. F. Grundriss der Pflanzengeographie, 12. 1836.
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8. Pfeffer-Ewart. Physiology of Plants, 1:13. 1900.
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10. Jevons, W. A. The Principles of Science, 2:137. 1874.
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1895.
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14. Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent., 18:311. 1904.
15. Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902.
16. Physiologische Pflanzenanatomie. 3d ed., 537. 1904.
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Abhandlungen über Pflanzenphysiologie. 1:355. 1892.
18. Clements, E. S. The Relation of Leaf Structure to Physical Factors. 1905.
19. De Vries, H. Die Mutationstheorie, 1:6. 1901.
20. Pound and Clements. A Method of Determining the Abundance of Secondary Species.
Minn. Bot. Studies, 2:19. 1898.
21. The Development and Structure of Vegetation, 84. 1904.
Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Neb., 5:29. 1901.
Mesophytes. Rep. Bot. Surv. Nebr., 6:48. 1902.
4. Bunsen, R., and Roscoe, H. Photometrische Untersuchungen. Poggendorff’s Annalen.,
117:529. 1862.
Wiesner, J.
Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete. Sitzb. Akad. Wiss.
Wien., I, 1893. II, 1895.
Untersuchungen über das photochemische Klima von Wien, Cairo, und Buitenzorg (Java)
Denksch. Kais. Akad. Wien., 64. 1896.
Untersuchungen über den Lichtgenuss der Pflanzen im arktischen Gebiete. Sitzb. Kais. Akad.
Wien., 109. 1900.
5. Reinke, J. Bot. Zeit., 41:713. 1883.
6. Meyen, F. J. F. Grundriss der Pflanzengeographie, 12. 1836.
7. Instructions to Field Parties and Descriptions of Soil Types, 35. 1903.
8. Pfeffer-Ewart. Physiology of Plants, 1:13. 1900.
9. The Relation of Leaf Structure to Physical Factors. 1905.
10. Jevons, W. A. The Principles of Science, 2:137. 1874.
11. Die Transpiration der Pflanzen, 14. 1904.
12. Recherches sur le Rôle Physiologique de l’Eau dans la Végétation. Ann. Nat. Sci., 7:20:65.
1895.
13. Observations on Stomata by a New Method. Proc. Camb. Phil. Soc., 9:303. 1897.
14. Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent., 18:311. 1904.
15. Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902.
16. Physiologische Pflanzenanatomie. 3d ed., 537. 1904.
17. Sachs, J. Ein Beitrag zur Kenntniss der Ernährungsthätigkeit der Blätter. Gesammelte
Abhandlungen über Pflanzenphysiologie. 1:355. 1892.
18. Clements, E. S. The Relation of Leaf Structure to Physical Factors. 1905.
19. De Vries, H. Die Mutationstheorie, 1:6. 1901.
20. Pound and Clements. A Method of Determining the Abundance of Secondary Species.
Minn. Bot. Studies, 2:19. 1898.
21. The Development and Structure of Vegetation, 84. 1904.
Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Neb., 5:29. 1901.
Page 438
22. Pound and Clements. The Vegetation Regions of the Prairie Province. Bot. Gaz., 25:381.
1898.
23. Clements, F. E. and E. S. Herbaria Formationum Coloradensium. 1902.
24. Lehrbuch der Ökologischen Pflanzengeographie, 97. 1896.
25. Phytogeography of Nebraska, 1st ed., 101. 1898.
26. Pflanzengeographie auf physiologischer Grundlage, 208. 1898.
27. Die Vegetation der Erde. Engler Bot. Jahrb., 17:b55. 1893.
28. Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 5:36, 46.
1901.
29. Grundzüge einer allgemeinen Pflanzengeographie, 157. 1823.
30. Essai de phytostatique, etc. 1849.
31. l. c., 116. 1896.
32. l. c., 3. 1898.
33. Uber mechanische Shutzmittel der Samen gegen schädliche Einflüsse von aussen. Engler
Bot. Jahrb., 5:56. 1883.
34. Essai Elémentaire de Géographie Botanique, 45. 1820.
35. Stationes Plantarum Amoen. Acad., 4:64. 1754.
36. Die Vegetation der Erde, 4. 1872.
37. When this word was first proposed, the author did not know that Briquet had already applied
the term polytopism to this concept (Ann. Conserv. Bot. Gen., 5:73. 1901). Since polygenesis
expresses the idea of origin, and applies to multiple origin in time as well as in space, it is retained as
the name of this concept. Polytopic and monotopic are adopted for multiple and single origin in space
respectively, and polychronic and monochronic are proposed for similar origin in time.
38. The Origin of Species, 186. 1859.
39. Rocks, Rock-weathering, and Soils, 300. 1897.
40. Relation d’un Voyage du Levant. 1717.
41. Clements, F. E. The Development and Structure of Vegetation. Rep. Bot. Surv. Nebr., 7:163.
1904.
42. Clements, F. E. The Development and Structure of Vegetation, 24, 27. 1904.
43. Clements, F. E. A System of Nomenclature for Phytogeography. Engler Jahrb., 31:b70:1.
1902.
1898.
23. Clements, F. E. and E. S. Herbaria Formationum Coloradensium. 1902.
24. Lehrbuch der Ökologischen Pflanzengeographie, 97. 1896.
25. Phytogeography of Nebraska, 1st ed., 101. 1898.
26. Pflanzengeographie auf physiologischer Grundlage, 208. 1898.
27. Die Vegetation der Erde. Engler Bot. Jahrb., 17:b55. 1893.
28. Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 5:36, 46.
1901.
29. Grundzüge einer allgemeinen Pflanzengeographie, 157. 1823.
30. Essai de phytostatique, etc. 1849.
31. l. c., 116. 1896.
32. l. c., 3. 1898.
33. Uber mechanische Shutzmittel der Samen gegen schädliche Einflüsse von aussen. Engler
Bot. Jahrb., 5:56. 1883.
34. Essai Elémentaire de Géographie Botanique, 45. 1820.
35. Stationes Plantarum Amoen. Acad., 4:64. 1754.
36. Die Vegetation der Erde, 4. 1872.
37. When this word was first proposed, the author did not know that Briquet had already applied
the term polytopism to this concept (Ann. Conserv. Bot. Gen., 5:73. 1901). Since polygenesis
expresses the idea of origin, and applies to multiple origin in time as well as in space, it is retained as
the name of this concept. Polytopic and monotopic are adopted for multiple and single origin in space
respectively, and polychronic and monochronic are proposed for similar origin in time.
38. The Origin of Species, 186. 1859.
39. Rocks, Rock-weathering, and Soils, 300. 1897.
40. Relation d’un Voyage du Levant. 1717.
41. Clements, F. E. The Development and Structure of Vegetation. Rep. Bot. Surv. Nebr., 7:163.
1904.
42. Clements, F. E. The Development and Structure of Vegetation, 24, 27. 1904.
43. Clements, F. E. A System of Nomenclature for Phytogeography. Engler Jahrb., 31:b70:1.
1902.
Page 439
44. The terms, oceanad, hylad, poad, eremad, etc., are proposed in place of oceanophyte,
hylophyte, etc. They are much shorter and make consistent groups under the general term, ecad., i. e.,
habitat form.
45. Clements, F. E. The Development and Structure of Vegetation, 166. 1904.
hylophyte, etc. They are much shorter and make consistent groups under the general term, ecad., i. e.,
habitat form.
45. Clements, F. E. The Development and Structure of Vegetation, 166. 1904.
Page 440
Page 441
TRANSCRIBER’S NOTES
Page Changed from Changed to
84 144 square inches of water, 144 cubic inches of water,
with an opening ¼ × 12 with an opening ¼ × 12
inches at the base inches at the base
234 as many readily be seen from as may readily be seen from
the fact that migration varies the fact that migration varies
inversely as the inversely as the
319 ochthi´um (ὄχφη, bank), a ochthi´um (ὄχθη, bank), a
bank formation; ochthad, a bank formation; ochthad, a
bank plant bank plant
1. Typos fixed; non-standard spelling and dialect retained.
Page Changed from Changed to
84 144 square inches of water, 144 cubic inches of water,
with an opening ¼ × 12 with an opening ¼ × 12
inches at the base inches at the base
234 as many readily be seen from as may readily be seen from
the fact that migration varies the fact that migration varies
inversely as the inversely as the
319 ochthi´um (ὄχφη, bank), a ochthi´um (ὄχθη, bank), a
bank formation; ochthad, a bank formation; ochthad, a
bank plant bank plant
1. Typos fixed; non-standard spelling and dialect retained.
Page 442
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